Human carcinogenic risk analysis and utilization of shale gas water-based drilling cuttings in road materials

Water-based drilling cuttings (WDC) generated during shale gas development will endanger human health and ecological security. The modern analytical techniques are used to analyze the organic pollutants in WDC, and the human health and ecological security risks of harmful pollutants in WDC under specific scenarios are evaluated. The results showed that the content of organic pollutants in WDC was evaluated by human health and safety risk assessment. The comprehensive carcinogenic risks of all exposure pathways of single pollutant benzo(a)anthracene, benzo(a)pyrene, benzo(k)fluoranthene, and indeno(1,2,3-cd)pyrene were acceptable. However, the cumulative carcinogenic risk of exposure to dibenzo(a,h)anthracene particles via skin exposure was not acceptable. It was considered that only dibenzo(a,h)anthracene had carcinogenic effect, and the risk control limit of dibenzo(a,h)anthracene in WDC was 1.8700 mg/kg by calculation. As well as, the “WDC-cement” gel composite structure was deeply analyzed, and the physical and chemical properties and mechanism of organic pollutants in cement solidified WDC were analyzed, which provided theoretical support for the study of WDC pavement cushion formula. Based on the above conclusions and combined with the actual site, by studying and adjusting the formula of WDC pavement cushion, the WDC pavement cushion was finally designed by 6% cement + 50% WDC + 44% crushed stone. The 7d unconfined compressive strength met the requirements of the Chinese standard “Technical Guidelines for Construction of Highway Roadbases” (JTG/T F20-2015). Also, the process route of WDC as road cushion product was sampled and analyzed. In addition, the leaching concentration of main pollutants all met the relevant standards of China. Therefore, this study can provide a favorable way for the efficient, safe, and environmentally friendly utilization of WDC, and ensure the ecological environment safety and human health safety of WDC in resource utilization.


Introduction
The outbreak of the coronavirus disease  in late 2019 has had a great impact on the global economy. Due to the close ties around the world, the COVID-19 epidemic leads to the global supply chain rupture, which leads to a new round of global energy crisis. The epidemic is also expected to have a lasting impact on future global energy needs (Wang and Su 2020;Hu et al. 2022). At the same time, human activities have intensified global climate change. If greenhouse gas emissions such as carbon dioxide are not strictly controlled, the global temperature in the twenty-first century will rise by more than 2 °C, which will have a serious impact on human society and the natural environment (Wang and Xiong 2021;Li et al. 2022). Reducing greenhouse gas emissions is the common responsibility of all mankind. Therefore, it is urgent to find an efficient and clean Responsible Editor: Guilherme L. Dotto * Kai Wu wukai@tongji.edu.cn 1 energy to meet the current global energy demand. With the increasing depletion of fossil fuels such as coal and oil, shale gas resources have broad development prospects worldwide.
China is a country with abundant shale gas resources. Shale gas refers to the natural gas enriched in the shale rich in organic matter, which is dominated by thermal maturity cracking or biological action and the interaction between the two. It has high calorific value and is a very efficient and green clean energy with broad market prospects and economic value (Li et al. 2020a, b;Wang et al. 2018a, b, c, d). However, in the process of shale gas development and utilization, large amounts of water-based drilling cuttings (WDC) are produced. It is necessary to use water-based drilling fluid to drill to about 1800 m in the drilling process of the second horizontal section of shale gas, and the waste returned to the ground is WDC Xiong 2021, Wang et al. 2014). Its composition mainly includes expansive soil, lubricant, pure alkali, KCl, and heavy metals. Its composition is complex, and easy to cause pollution to the environment. Therefore, it is urgent to dispose of WDC safely and environmentally. At present, some studies only focus on the alternative methods of resource utilization of these wastes in the resource utilization of WDC, such as the production of building materials, such as bricks (George and Antonis 2021;Mojtaba et al. 2020;Symeonides et al. 2019;Ayati et al. 2019;Dai et al. 2019), concrete Mohsen et al. 2021;Li et al. 2020a, b;Amelung al. 2020;Ma et al. 2019), aggregates (Bamdad et al. 2019;Siddique et al. 2021;Sundis et al. 2021;Moreno-Maroto et al. 2018;Prachasaree et al. 2020), glass materials (Baino and Ferraris 2019;Stoch et al. 2018;Grilo et al. 2019;Alves et al. 2017), slag-red mud cementitious materials (Ban et al. 2018;Faried et al. 2021;Mymrin et al. 2017;Ye et al. 2017), and ceramic materials Cheng et al. 2018;Ren et al. 2019;Cao et al. 2019).
The study of sintered bricks is to use WDC as a partial substitute material and heat treatment method to heat the waste at high temperature to induce coherent bonding or welding of adjacent particles, so as to gain dense products with lower porosity and make the physical, mechanical, and environmental properties of WDC sintered bricks meet the corresponding Chinese standards and ASTM standards requirements Li et al. 2019;Luo et al. 2020). The research of concrete is mainly based on the physical and chemical properties of WDC. Shale gas WDC is used as siliceous and calcareous materials to improve the physical and chemical properties of concrete. At the same time, a new WDC resource utilization technology is developed (Wang and Xiong 2021;Ghorbani et al. 2019;Jiang et al. 2020). The research on aggregate mainly focuses on the applicability of different types of secondary materials as artificial lightweight aggregate. In addition to the commonly used materials (such as clay and fly ash) for aggregate production, various types of waste can be used as alternative materials (Piszcz-Karaś et al. 2019;Burciaga-Díaz et al. 2020;Amin et al. 2021). The research of glass materials is mainly based on the mixture of kaolin and oil well drilling waste as raw materials. The recycled raw materials (waste) are mixed with commercial materials to adjust the composition of glass, and glass-ceramic precursor materials are obtained by melting. The obtained vitreous powder is studied by thermal, chemical, and structural characterization techniques. The pellets were pressed and sintered to obtain glass-ceramic materials (Danielle et al. 2019;Stoch et al. 2018;Mohajerani et al. 2019). The research on slag-red mud cementing material mainly uses waste drilling fluid, blast furnace slag, and red mud as raw materials to prepare a new type of cementing material, aiming at achieving an environmentally friendly and efficient solution for waste recycling. Meanwhile, the physical and chemical properties of slag-red mud cementitious materials were evaluated by compressive strength, X-ray diffraction analysis, and mercury intrusion porosimetry (Cheng et al. 2019;Wang et al. 2018a, b, c, d).
In the study of ceramic materials, spinel-corundum ceramics were prepared from ferrochrome slag and bauxite. The sintering behavior of spinel corundum ceramics is preliminarily analyzed. In addition, the liquid phase process is modified by adding pyrolusite in the ceramic system to obtain high strength ceramics (Shao et al. 2022;Fan et al. 2021). In summary, the above studies added binders (such as Portland cement, fly ash, and lime) to WDC to effectively fix pollutants through chemical fixation and physical coating. The cured WDC is buried on site or transferred to other sites for final disposal. Obviously, this process is mainly a process of fixing pollutants rather than completely damaging pollutants, which may release and pollute soil and groundwater in the future.
Although domestic and international conventional oilfield WDC (Abdul-Wahab et al. 2020;Burghard et al. 2021;Xie et al. 2020;Maziar et al. 2018;Notani et al. 2019, Bedia et al. 2018Burciaga-Díaz et al. 2020;Wang et al. 2017a, b;Allwood 2018;Ghorbani et al. 2019) has a lot of research on resource utilization, but there are still many problems in risk control of resource products. (1) Presently, there is no uniform and systematic testing method for resource products.
(2) There is no unified pollution control standard for comprehensive utilization of shale gas cuttings. (3) Longterm use safety performance of resource products cannot be guaranteed. Moreover, the physicochemical properties and pollution characteristics of shale gas WDC are different from those of ordinary oilfield WDC, so it cannot play a targeted guiding role in the resource utilization of shale gas WDC. Based on the above problems of resource products, this paper puts forward reasonable suggestions.
Therefore, the innovation of this paper is to use the human health safety risk assessment method to evaluate the safety of organic pollutants in WDC, and to construct the evaluation model of human health and ecological safety risk assessment. In this paper, the harmful organic pollutants in WDC solid phase were systematically studied and analyzed by modern analysis technology, and the release and migration characteristics of organic pollutants in WDC in various environmental media were comprehensively analyzed. The human health and safety risk assessment method was used to evaluate the safety of organic pollutants in WDC, and the evaluation model of human health and ecological safety risk assessment was constructed. The pollution control indexes and limits of WDC resource utilization were obtained by calculation, which laid a solid foundation for its resource utilization. Based on the above research conclusions and filed reality, a method for efficient utilization of shale gas WDC road materials is proposed, and the curing mechanism of "WDC-cement" gel composite structure in WDC road materials is deeply analyzed to further improve the environmental safety of WDC resource products. The above research results will further promote the basic technology research of shale gas WDC resource utilization, and provide a favorable way for the efficient, safe, and environmentally friendly utilization of WDC.

Raw materials
The Portland cement 42.5 (P.O 42.5) is available from a Chongqing Cement Co., Ltd (China). WDC is in a powdered form (after drying) and the particle medium diameter is 42 µm, and is given by a shale gas field in Chongqing city, China.
The selection of WDC samples fully considers the characteristics of solid waste raw materials of drilling cuttings in different regions. According to the requirements of Chinese standard Technical specifications on identification for hazardous waste (HJ/T 298-2019), the sampling quantity is determined according to the total amount of WDC, and the requirements of sample quantity are met.

Detection of organic pollutants in WDC
At present, the determination methods of polycyclic aromatic hydrocarbons in soil mainly include liquid chromatography, gas chromatography mass spectrometry, and other methods. Due to the high sensitivity and strong anti-interference ability of gas chromatography mass spectrometry, it is listed as the designated method of national soil screening. Therefore, this study used Chinese standard Water quality-Determination of volatile organic compounds-Headspace/ Gas chromatography mass spectrometry (HJ 810-2016) to detect the organic content in WDC. One thousand milligrams per liter standard substances, including benzo(a)anthracene, benzo(k)fluoranthene, benzo(a)pyrene, indeno(1,2,3-cd)pyrene, and dibenzo(a,h)anthracene, Beijing Tanmo Quality Inspection Technology Co., Ltd., all other reagents used are analytically pure.

Sample pretreatment and determination
After preliminary screening, the collected WDC samples were mixed with a certain amount of standard reserve liquid and stirred evenly, and then placed in a grinding brown glass bottle for sealing and avoiding light preservation. The preservation temperature was 4 °C and the shelf life was 10 days.
The 25-g WDC sample was placed in 250-mL glass triangular beaker, adding 40 mL acetonitrile ultrasonic 10 min, standing 15 min, transfer the upper liquid placed in 50-mL plastic centrifuge tube, 5000 r/min centrifugal 5 min, the supernatant after centrifugation through the anhydrous sodium sulfate device, filtration, dehydration, collection of filtrate, circulation 2 times. The filtrate collected twice was concentrated to dryness by rotary evaporator, and 100-µL methanol was added to constant volume. The filtrate was transferred to the injection bottle with an inner liner for GC/ MS analysis.

Human health risk assessment of WDC organic pollutants
The Chinese standard Technical guidelines for risk assessment of soil contamination of land for construcion (HJ 25.3-2019) was used for risk assessment and risk control value calculation of organic pollutants in WDC. The calculation formula is as follows:

Calculation of organic matter exposure in WDC
(1) Intake of WDC particles through mouth The exposure corresponding to the oral intake of solid waste particles is calculated by Formula (1): where OISERca is the exposure to oral intake of solid waste particles (carcinogenic effect), kg(soil)•kg −1 (body weight)•d −1 . Tca is the average time of carcinogenic effect, d; OSIRa is the daily soil intake by adults, mg•d −1 . EDa is the adult exposure cycle, a. EFa is the adult exposure frequency, d•a −1 . BWa is the larger weight, kg. ABSo is the oral absorption efficiency factor, dimensionless.
(2) Calculation of particle exposure of skin contact WDC The exposure amount corresponding to the skin contact with WDC particle pathway is calculated by Formula (2): where DCSERca is the exposure of debris particles in skin contact with WDC (carcinogenic effect), kg(soil)•kg −1 (body weight)•d −1 . ATca is the average time of carcinogenic effect, d; for other parameters, see Formula (1), and SAEa is the parameter values calculated using Formula (3): where Ha is the average height of adults, cm, and SERa is the adult exposed skin area ratio, dimensionless.
(3) Inhalation of dust particles from WDC by breathing The amount of soil exposure corresponding to inhalation of soil particulate matter is calculated by Formula (4): where PISERca is the soil exposure to inhaled soil particles (carcinogenic effect), kg(soil)•kg −1 (body weight)•d −1 . PM10 is the inhalable particulate matter content in air, mg•m −3 . DAIRa is the daily air respiration of adults, m 3 •d −1 . PIAF is the inhaled soil particulate matter in vivo retention ratio, dimensionless. Fspi is the proportion of particulate matter from soil in indoor air, dimensionless. Fspo is the proportion of particulate matter from soil in outdoor air, dimensionless. EFIa is the indoor exposure frequency of adults, d•a −1 . EFOa is the outdoor exposure frequency of adults, d•a −1 . ATca is the average time of carcinogenic effect, d.
(4) Drinking groundwater The groundwater exposure corresponding to the drinking ground water pathway is calculated by Formula (5): where CGWERca is the exposure to groundwater corresponding to drinking affected groundwater (carcinogenic effect), L(groundwater)•kg −1 (body weight)•d −1 . ATca is the average time of carcinogenic effect, d.

(a) Oral intake of WDC particles
The carcinogenic risk of single pollutant in the WDC particles is calculated by Formula (6): where CR ois is the oral intake of WDC particles exposed to a single pollutant carcinogenic risk, dimensionless. SFo is the oral intake carcinogenic slope factor, (mg•kg −1 •d −1 ) −1 . C is the oral intake carcinogenic slope factor, mg•kg −1 .

(b) Skin contact with WDC particles
The carcinogenic risk of skin contacting with single pollutant in WDC and particles is calculated by Formula (7): where CR DCS is the carcinogenic risk of single pollutant exposure to WDC particles in skin, dimensionless. SFd is the skin contact carcinogenic slope factor, (mg•kg −1 •d −1 ) −1 .
The carcinogenic risk of single pollutant in inhaled dust particles is calculated by Formulas (8) and (9): where CR PIS is the inhaled dust particles exposed to a single pollutant carcinogenic risk, dimensionless. SFi is the respiratory inhalation carcinogenic slope factor, (mg•kg −1 •d −1 ) −1 . IUR is the unit carcinogenic risk of respiratory inhalation, m 3 •mg −1 .
(iv) Drinking groundwater For the risk assessment model of drinking groundwater, according to the actual situation of field investigation, some contents refer to the site pollution risk assessment model of the Chinese standard Technical guidelines for risk assessment of soil contamination of land for construcion (HJ 25.3-2019) and RBCA in the USA.
The process of WDC leaching to groundwater can be calculated according to the following Formula (10): where LF gw is the soil leaching dilution factor, kg/L. U gw is the Darcy rate of groundwater, cm•a −1 . δ gw is the thickness of groundwater mixed zone, cm. I f is the net infiltration rate of rainwater, cm•a −1 . H′ is Henry's constant, dimensionless. θ avs is the volume ratio of pore air in unsaturated soil layer, dimensionless. θ wvs is the volume ratio of pore water in unsaturated soil layer, dimensionless. K s is the distribution coefficient of pollutants in soil and water L•kg −1 , organic matter K s , and inorganic matter K s = K d . ρ b is the volume weight of soil, kg•L −1 . W gw is the length of contaminated area parallel to groundwater flow, cm.
where K oc is the distribution coefficient of soil organic carton/ soil pore water, L•kg −1 . f oc is the soil organic carbon mass fraction, dimensionless. f om is the organic matter content, g•kg −1 .
After rainwater leaches into the bedding area of WDC field, the pollutants are dissolved and enter the groundwater with the leaching solution. The concentration of groundwater pollutants below this area can be calculated according to Formula (13): where C gw is the concentrations of pollutants in groundwater, mg/L. LF gw is the soil leaching dilution factor, kg/L. C is concentration of pollutants in WDC, mg•kg −1 .
The calculation of carcinogenic risk is presented by Formula (14): where CRcgw is the cancer risk of drinking groundwater, dimensionless. SFo is the oral intake carcinogenic slope factor, (mg•kg −1 •d −1 ) −1 .

Calculation of soil risk control value of heavy metals in WDC
Based on the soil risk control value of carcinogenic effect by oral intake of soil, Formula (15) is used to calculate.
where RCVS ois is the soil risk control value based on carcinogenic effect of oral intake of soil pathway, mg•kg −1 . OISER ca is the exposure to oral intake of solid waste particles (carcinogenic effect), kg(soil)•kg −1 (body weight)•d −1 . ACR is the acceptable carcinogenic risk, dimensionless, value 10 −5 . SF o is the oral intake carcinogenic slope factor, (mg•kg −1 •d −1 ) −1 .
Based on the soil risk control value of carcinogenic effect of skin contact with soil, Formula (16) is used to calculate.
where RCVS dcs is the soil risk control value based on carcinogenic effect of skin contact with soil pathway, mg•kg −1 . DCSER ca is the exposure of debris particles in skin contact with WDC (carcinogenic effect), kg(soil)•kg −1 (body weight)•d −1 . ACR is the acceptable carcinogenic risk, dimensionless, value 10 −5 . SF d is the oral intake carcinogenic slope factor, (mg•kg −1 •d −1 ) −1 .
Based on the soil risk control value of carcinogenic effect by inhalation of soil particles, Formula (17) is used to calculate.
where RCVS dcs is the soil risk control value based on carcinogenic effect of skin contact with soil pathway, mg•kg −1 . PISER ca is the exposure of inhaled dust particles (carcinogenic effect), kg(soil)•kg −1 (body weight)•d −1 . ACR is the acceptable carcinogenic risk, dimensionless, value 10 −5 . SF i is the respiratory inhalation carcinogenic slope factor, (mg•kg −1 •d −1 ) −1 .
Based on the soil risk control values of comprehensive carcinogenic effects of three exposure pathways, Formula (18) is calculated according to Technical guidelines for risk assessment of soil contamination of land for construcion (HJ 25.3-2019).
where RCVS n is the soil risk control value of single pollutant (n) based on comprehensive carcinogenic effects of three soil exposure pathways, mg•kg −1 .

The radioactive calculation formula of building materials
where C Ra , C Th , and C K are the specific radioactivity (Bq/ Kg) of natural radionuclides radium-226, thorium-232, and potassium-40 in building materials, respectively. According to the requirements of Chinese standard limits of radionuclides in building materials (GB6566-2010), after testing, when WDC are used as building materials, their I Ra and I r indexes need to be less than or equal to 1.0. According to the limits of radionuclides in building materials (GB 6566-2010), the use range of WDC is not limited.

Chemical composition
The chemical analysis of raw materials was distinguished by RIGAKU ZSX Priums up-irradiation X-ray fluorescence spectrometer produced in Japan. The power of X-ray tube is 4 kW.

XRD (X-ray diffraction analysis)
The mineral crystal phase of WDC and its hydrated products were tested by using Netherlands X'Pert Pro XRD, step width is 2 × 0.02°/step, and scanning speed is 80 mm/min.

SEM (scanning electron microscopy)
Microstructure is analyzed with a scanning electron microscope (SEM, ASTEREO SCAN440, Leica Cambridge Ltd) which be used to investigate the morphology and the elemental analysis.

FT-IR (Fourier transform infrared)
The hydration product's chemical bonds of the samples were analyzed by Fourier transform infrared spectroscopy (FT-IR). The readily available for testing powder blended with KBr is then squeezed at 2000 psi for 5 min for analysis; frequency range is 4 × 10 2 , 4 × 10 3 cm, and 0.5 cm resolution ratio.

Results and discussion
Based on the basic physical and chemical properties of WDC samples, this study comprehensively analyzed the release and migration characteristics of organic pollutants in WDC in various environmental media. The human health and safety risk assessment method was innovatively used to evaluate the safety of organic pollutants in WDC, and the human health and ecological safety risk assessment model was constructed. The pollution control indexes and limits of WDC resource utilization were obtained by calculation. Reasonable control means and resource utilization direction were selected to treat WDC (Loizia et al. 2021a, b), and the safety control mechanism of organic pollutants in resource products was analyzed to comprehensively guarantee the environmental safety and human health of WDC resource products in use.

Risk assessment of organic pollutants in WDC
According to the leaching toxicity test results of WDC in Table 22, all the detected substances do not exceed the limit value in Identification standards for hazardous wastes-Identification for extraction toxicity (GB5085.3-2007). The detected heavy metals do not exceed the standard values of residential land in Risk screening guideline values for soil contamination of development land (three drafts) (HJ 25.5-2015) and Soil environmental quality-Risk control standard for soil contamination of agricultural land (GB 15,618-2018). The detection concentrations of benzo(a) pyrene are all higher than the concentration limit, and the maximum exceeding multiple is 2.37 times, which is the main exceeding factor.

Primary selection of organic pollutants in WDC
WDC were leached by Water quality-Determination of volatile organic compounds-Headspace /Gas chromatography mass spectrometry (HJ 810-2016) Table 1.
Benzo(a)anthracene, benzo(k)fluoranthene, benzo(a) pyrene, indeno(1,2,3-cd)pyrene, and dibenzo(a,h)anthracene exceed the standard values of residential land in Risk screening guideline values for soil contamination of development land (three drafts) (HJ 25.5-2015) and Soil environmental quality-Risk control standard for soil contamination of agricultural land (GB 15,618-2018). The maximum exceeding multiples of residential land standards exceeding Risk screening guideline values for soil contamination of development land (three drafts) (HJ 25.5-2015) are 3.49, 7.36, 8.46, 1.06, and 31.34 times, respectively. And benzo(a)pyrene also exceeds Soil environmental quality-Risk control standard for soil contamination of agricultural land (GB 15,618-2018); the maximum exceeding multiple is 15.07 times. Other indicators do not exceed the corresponding standards, so benzo(a)anthracene, benzo(k) fluoranthene, benzo(a)pyrene, indeno(1,2,3-cd)pyrene, and dibenzo(a,h)anthracene are the main exceeding factors, and the specific exceeding conditions are shown in Table 20 in Annex 2. Therefore, benzo(a)anthracene, benzo(a)pyrene, indeno(1,2,3-cd)pyrene, and dibenzo(a,h)anthracene in the solid phase of WDC are the main control factors (Wang et al. 2018a, b, c, d;Rehman et al. 2017). The leaching results of organic pollutants and the main exceeding factors are shown in Table 2.

Risk control calculation of comprehensive utilization index of WDC
The solid phase of WDC is used to lay the foundation materials of the non-sensitive regional open well road and bedding well site. In order to ensure the human health of the site, it is necessary to carry out the risk assessment and determine the risk control value of the site soil. Therefore, the Chinese standard Technical guidelines for risk assessment of soil contamination of land for construcion (HJ 25.3-2019) is used for risk assessment and risk control value calculation of organic pollutants in WDC. The whole assessment process mainly includes hazard identification, toxicity assessment, exposure assessment, risk representation, and control value calculation (Boulicault et al. 2016;Tsangas et al. 2019).
Hazard identification When the solid phase comprehensive utilization of WDC generated during shale gas development is set to be used for bedding and road construction in the well site, the comprehensive utilization area is located around the drilling platforms in Chongqing area, belonging to industrial land, so the possible sensitive receptors are mainly human body and soil (Davarpanah et al. 2018).
Toxicity assessment On the basis of hazard identification, this study considered the impact of pollutants in WDC on human health through various migration pathways after cushion utilization and road construction in the region. Specific content of solid waste toxicity assessment includes analysis of health effects (including carcinogenic and noncarcinogenic effects) and determination of toxic parameter values of pollutants. The toxicity parameters used in this study are shown in Table 21 in Annex 2. The toxicity parameters are derived from the database of RBCA site risk assessment software and the data collation of solids from the Chinese Academy of Environmental Sciences.
Exposure assessment Exposure assessment is to use the model to predict the migration and transformation process of pollutants from source to medium, and analyze the spatial distribution and release concentration of pollutants related to the evaluation area. That is to say, the process of measurement, estimation or prediction of exposure amount, exposure frequency, exposure duration, and exposure pathway under the condition of target harmful pollutants or physical factors is the quantitative basis for risk assessment (Mojoudi et al. 2019). Overall, exposure assessment includes three aspects: analysis of exposure scenarios, determination of exposure pathways, and exposure calculation (Fig. 1).
a. Analysis of exposure scenarios WDC are used as well site bedding area to plan drilling platforms (industrial land) in Chongqing area. Combined with field survey results, the following evaluation exposure scenarios are established. Due to the existence of sensitive people in the park, considering the influence of well site bedding on this part of the sensitive population, the standard well site of 4 wells/platform is selected as the scene of this risk assessment.
b. Determination of exposure pathways Exposure pathways are closely related to pollution sources, location, environmental release types, population location, and activity patterns, including four elements: sources and chemical release mechanisms, retention or transport media (or chemical transport media), exposure pathways of potential populations to pollution media, and contact points. According to the analysis of exposure pathways in this study, the exposure pathways of pollutants to human body include skin absorption, respiratory system inhalation, and digestive system intake (Salari et al. 2019). The exposure pathway analysis and schematic diagram of WDC pollutants are shown in Table 3, respectively. The specific exposure pathways can be refined as follows: (1) platform workers inadvertently intake solid waste particles at work; (2) the skin of platform workers exposed to solid waste particles at work; (3) platform workers breathe in solid waste dust particles; (4) platform workers directly drink groundwater below the area.
iii. Calculation of exposure Exposure parameters include exposure frequency, exposure period, intake of pollutants, and related parameters of human body. According to the field investigation results and related data of shale gas field operation area, the exposure parameters are selected with reference to the Technical guidelines for risk assessment of soil contamination of land for construcion (HJ 25.3-2019) and RBCA site risk assessment software (Fu et al. 2019;Issabayeva et al. 2018). Some of the data are estimated according to the actual situation of the site. At the same time, in the selection of exposure parameters, we consider the principle of limited adverse conditions hypothesis, and evaluate the potential harm to the human body and the surrounding environment under the adverse conditions. The specific parameters are shown in Table 22 in Annex 2. Because PAHs are carcinogenic pollutants, we only need to consider the carcinogenic effect exposure of a single pollutant.

Reselection of organic pollutants from WDC
Calculation of organic matter exposure in WDC The main pollutants concerned in the evaluation of WDC/mud are five polycyclic aromatic hydrocarbons (PAHs), namely, anthracene, benzo(k)fluoranthene, benzo(a)pyrene, dibenzo(a,h) anthracene, and indeno(1,2,3-cd)pyrene. The toxicity parameters of major PAHs are shown in Table 21 in Annex 2. In the scenario of WDC well site bedding and other utilization, it is concerned that pollutants are carcinogenic substances. In the domestic wind assessment guidelines, there are corresponding ways of carcinogenic risk slope factor parameters, so only the carcinogenic risk of each pollutant is calculated. In the calculation, the maximum average concentration is used as the pollution source concentration for risk assessment, that is, the overall risk level is estimated in a relatively conservative way (Okparanma et al. 2018).

i) Intake of WDC particles through mouth
For the carcinogenic effect of a single pollutant, consider the lifetime hazards of exposure in adults. The calculation formula is shown in Formula (1). The calculation results of exposure amount of WDC through mouth are shown in Table 4. It can be seen from the table that the exposure amount (carcinogenic effect) of WDC taken orally is 4.19 × 10 −7 kg•kg −1 •d −1 . ii) Calculation of particle exposure of skin contact WDC For the carcinogenic effect of single pollutant, consider the lifelong hazard of human exposure in adulthood. The calculation formula is shown in Formula (2). The calculation results of debris particle exposure in skin contact WDC are shown in Table 5. It can be seen from the table that the exposure amount (carcinogenic effect) of skin contacting WDC particles is 3.11 × 10 −7 kg•kg −1 •d −1 .

iii) Inhalation of dust particles from WDC by breathing
In industrial and other land use, humans can breathe in indoor and outdoor air from soil particles exposed to soil pollutants. For carcinogenic effects of single pollutants, the lifetime hazards of exposure in adults are considered. The calculation formula is shown in Formula (4). The calculation results of dust particles from WDC by breathing are shown in Table 6. It can be seen from the table that the

iv) Drinking groundwater
For the carcinogenic effect of a single pollutant, consider the exposure hazards of the population in adulthood. The calculation formula is shown in Formula (5).
The calculation results of exposure of drinking groundwater are shown in Table 7. The table shows that the exposure (carcinogenic effect) of drinking groundwater is 4.19 × 10 −3 L•kg −1 •d −1 .
Calculation of carcinogenic effect of WDC. The main pollutants in the evaluation of WDC mud are benzo(a)anthracene, benzo(k)fluoranthene, benzo(a) pyrene, dibenzo(a,h)anthracene, and indeno(1,2,3-cd)    Table 21 in Annex 2. Referring to the Chinese standard of Soil environmental quality-Risk control standard for soil contamination of agricultural land (GB 15,618-2018), in the scenario of WDC well site bedding and other utilization, it is concerned that pollutants are carcinogenic substances. In the domestic wind assessment, there are corresponding carcinogenic risk slope factor parameters, so only the carcinogenic risk of each pollutant is calculated. In the calculation, the maximum average concentration is used as the pollution source concentration for risk assessment, that is, the overall risk level is estimated in a relatively conservative way (Elmouwahidi et al. 2017).
(1) Oral intake of WDC particles The calculating carcinogenic effects of oral WDC granules are presented in Formula (6). The risk calculation results of WDC intake through mouth are shown in Table 8.
(2) Skin contact with WDC particles The carcinogenic effect of skin contact with WDC particles is presented in Formula (7). The risk calculation results of skin contact with WDC particles are shown in Table 9.
(3) Respiration inhalation of WDC dust particles The carcinogenic effect of breathing inhaled WDC dust particles is presented in Formulas (8) and (9). The risk calculation results of inhalation of WDC and dust particles are shown in Table 10.
(4) Drinking groundwater WDC are leached and soaked by rainwater after bedding utilization. The pollutants in WDC dissolve with rainwater and diffuse into groundwater with the migration and diffusion of leaching solution (Voukali et al. 2021;Sean et al. 2021). The schematic diagram of leaching process is shown in Fig. 2.
In this study, according to the actual situation of field investigation, some contents refer to the site pollution risk assessment model of the Chinese standard Technical guidelines for risk assessment of soil contamination of land for construcion (HJ 25.3-2019) and RBCA in the USA. The process of WDC leaching to groundwater can be calculated according to Formula (10). The calculation results of leaching dilution factor LF gw in the bedding utilization process of WDC field are shown in Table 17 in Annex 1.
After rainwater leaches into the bedding area of WDC field, the pollutants are dissolved and enter the groundwater with the leaching solution. The concentration of groundwater pollutants below this area can be calculated according to Formula (13). The calculation of carcinogenic risk is  (14). The risk calculation results of drinking groundwater in WDC field are shown in Table 18 in Annex 1. Through the risk calculation, it can be seen that the cumulative carcinogenic risk of organic matter in the bedding utilization process of WDC well site is shown in Table 11. It can be seen that the cumulative carcinogenic risk of dibenzo(a,h) anthracene reaches 3.17E-05, which exceeds that of Chongqing in Technical guidelines for risk assessment of soil contamination of land for construcion (HJ 25.3-2019). The acceptable risk level of contaminated site is the cumulative carcinogenic risk value of 10 −5 , so it is not acceptable.
Through the above environmental risk calculation, the following conclusions can be drawn, in view of the carcinogenic risk: the cumulative carcinogenic risk of particle intake and skin contact pathway is at an unacceptable level, but the risk level of particle intake and skin contact is relatively low, and the corresponding environmental risk can be controlled by certain preventive measures, and the cumulative carcinogenic risk of dust inhalation and drinking groundwater is at an acceptable level. The comprehensive carcinogenic risk of all exposure pathways of single pollutant benzo(a)anthracene, benzo(a)pyrene, benzo(k)fluoranthene, and indeno(1,2,3-cd)pyrene is acceptable. The cumulative carcinogenic risks of particle intake of dibenzo(a,h) anthracene and skin contact pathway are at an unacceptable level.
Therefore, in view of the carcinogenic risk of dibenzo(a,h)anthracene, the soil limit calculation results of the comprehensive carcinogenic risk of all exposure pathways are shown in Table 11. According to the most unfavorable principle, the exposure amount of inhaled dust particles is calculated by the exposure amount (maximum) at 100 m downwind of oil-based ash (Demetriou et al. 2021).

Calculation of soil risk control value of organic matter in WDC
The calculation results of soil risk control values based on carcinogenic effects under different exposure conditions are shown in Table 19 in Annex 1.
Based on the combined carcinogenic effects of three exposure pathways, the soil risk control values were calculated by Formula (18). The detailed calculation process is shown in Table 12.
Based on the soil limits of carcinogenic risk calculated by Formula (18), the recommended control value of pollutant concentration is selected.
According to the risk assessment results shown in Table 13, the dibenzo(a,h)anthracene in WDC is required to be less than 1.8700 mg/kg under the environment of laying open well path and bedding well site with WDC.

Macroscopic analysis of WDC road cushion
The contents of toxic substances in WDC collected from A and B drilling platforms are analyzed, and the results are shown in Table 20 in Annex 2. Figure 3 shows the experimental results of WDC samples with different cement contents soaked for 28 days and 128 days respectively. The  Wang et al. 2018a, b, c, d).
The 7-day unconfined compressive strength of WDC specimens with different cement contents is shown in Fig. 4. Obviously, when the cement content is 6%, the unconfined compressive strength of 7 days reaches 3.1 MPa, which meets the requirements of the Chinese standard Technical Guidelines for Construction of Highway Roadbases (JTG/T F20-2015). When the content is 8%, the strength of the specimen decreases. This is because the C-S-H gel generated by cement hydration in the cementitious reaction wrapped WDC particles, which hindered the hydration of WDC particles and reduced the strength of the specimen. When the cement content was 10%, more C-S-H gels and AFt crystals were produced by cement hydration, which increased the strength of the specimen. Therefore, when the cement content is 10% and 12%, the strength of the specimen is higher. However, the cement content of 6% has met the standard requirements, in order to reduce the economic cost of construction and reduce carbon emissions. Therefore, when WDC are used as road cushion, the selected cement mass is 6%. Preliminary trial with WDC for road cushion specific formula is as follows: visible WDC content of 50% of the 7-day unconfined compressive strength of 3.1 MPa, if continue to increase the dosage of its early compressive strength, qualification rate is difficult to guarantee, so the maximum dosage of 50%. As shown in Table 14, the 7-day unconfined compressive strength of the cushion with 50% WDC basically meets the design requirements. So when designing road cushion, the cement content in the formula should not be more than 8% and WDC content should not be higher than 50% by using 42.5 ordinary Portland cement supplemented by part of the debris and water-mixed particles of WDC (Ayati et al. 2019;Senneca et al. 2020). In the construction process, 20 t and above static pressure rollers should be used, and the final well site cushion compaction degree should not be less than 93%, and the 7-day unconfined compressive strength should not be less than 3.0 MPa.

Microscopic analysis of WDC road cushion
Fourier transform infrared (FT-IR) can give the hint of bending vibration and stretching of Al-O and Si-O whose vibration frequencies increase with the increase of polymerization degree. Figure 5 shows the infrared spectrum of the samples with different cement ratios. The results show that the infrared spectrum curves of the samples with cement ratios of 6%, 8%, 10%, and 12% are roughly the same, indicating that the hydration products of the WDC samples do not change significantly at the early and late stages of hydration . The results show that the infrared absorption peak at 1034.8 cm −1 is the corresponding of the symmetric and asymmetric stretching vibration of sulfate in CaSO 4 . The absorption peak of 3442.9 cm −1 corresponds to the bending and stretching vibrations of H-O in Ca(OH) 2 . The characteristic absorption peaks of ettringite and calcium aluminate hydrate are 1112.0 cm −1 and 1419.7 cm −1 , respectively. The absorption peak intensity of AFt and calcium aluminate hydrate increased slightly with the increase of cement content from 6 to 10%. If the cement content exceeds 10%, they will fall sharply. In addition, the absorption peak intensity of Ca(OH) 2 decreases with the increase of WDC content. It is directly proved that the volcanic ash activity of WDC is caused by the active reaction of CaO with SiO 2 and the active reaction of gypsum and CaO with Al 2 O 3 to form ettringite (Han et al. 2020;Hossain et al. 2020).   Based on the curing principle of ettringite and C-S-H colloid generated during cement hydration on organic pollutants in WDC, in order to further study the role of WDC in road buffer system, cement with different contents is mixed with WDC and fine aggregate, and the hydration products of the system were studied by WDC (Senneca et al. 2020;Foroutan et al. 2018). The results show that the main crystalline phases identified in the hardened slurry of road cushion are ettringite, quartz, CaCO 3 , Ca(OH) 2 , and CaSO 4 . As shown in Fig. 6, with the addition of WDC, the crystal phase difference in XRD is large, and the intensity of diffraction peak at 47.8 is significantly reduced or even disappeared, indicating that there were more hydration products generated in the hydration process of WDC dissolved active alumina and silicate cement (Hossain et al. 2020). Secondary hydration products such as AFt and C-S-H gel could fill the gaps and pores between fine aggregate and gel particles, forming a more compact system. In addition, as the cement content increases from 6 to 10%, the diffraction peak of AFt gradually increases, but when the cement content reaches 12%, the diffraction peak intensity of AFt decreases slightly (Ciotoli et al. 2020;Antoniou and Antoniou 2019). Based on the basic principle of the reaction of organic pollutants in WDC with cement, in order to better understand the interaction between WDC and volcanic ash materials, road cushion samples with different cement ratios were used to observe the microstructure of early hydration under SEM. The results are shown in Fig. 7, which are the microstructure of early hydration products of WDC samples when the cement ratios are 6%, 8%, 10%, and 12%, respectively. It can be seen from the figure that the particles of WDC are irregular, the particle surface is porous and loose, and the WDC particles are mostly lamellar (Okparanma et al. 2018;Demetriou et al. 2021). With the increase of cement content, the porosity of WDC particles decreases significantly. It indicates that doping appropriate WDC is conducive to the densification of the early microstructure of the system, and the existence of C-S-H and Aft generated by the secondary hydration of WDC is also confirmed by XRD analysis Faried et al. 2021). The results show that the volcanic ash reaction of WDC generates a large number of AFt crystals interspersed in the C-S-H gel, so that the pores are filled to form a more compact microstructure. Therefore, under the same curing conditions, the microstructure of WDC sample is significantly optimized, so that it has higher durability and mechanical properties (Ghorbani et al. 2019;Jiang et al. 2020).

Environmental safety analysis of WDC cushion
The text results of WDC as cushion leaching solution are shown in Table 15. According to the text values of toxic substances in WDC as road cushion materials, the test results are analyzed. It can be obviously found that all the contents of substances Fig. 7 SEM morphology of a WDC particle products with cement content of 6%, b WDC particle products with cement content of 8%, c WDC particle products with cement content of 10%, d WDC particle products with cement content of 12%  (GB 5084-2021). At the same time, the contents of pollutants meet the standard limits of class I surface water in Environmental quality standards for surface water (GB 3838-2002), and the contents of heavy metals detected do not exceed the maximum allowable emission concentration of class I pollutants in Integrated wastewater discharge standard (GB 8978-1996). Therefore, WDC as road cushion materials meet the standard requirements (Han et al. 2020;Fu et al. 2019). By comparing and analyzing the WDC to meet the reference limit of heavy metal content in raw materials of cement kiln cooperative disposal of solid waste into the kiln, at the same time as building materials, shale gas WDC as raw materials of cement industry must also meet the national standard of radioactivity of building materials (Allwood 2018). This index is usually used to measure the radioactivity of building materials by internal irradiation index I Ra and external irradiation index I r . The calculation formula is shown in (19) and (20). The results are shown in Table 16.

Mechanism analysis of cement solidifying organic pollutants in road cushion with WDC
Cement solidification is a common method to convert toxic and hazardous solid waste into non-hazardous substances. Cement is the most commonly used stabilizer for hazardous waste. The technology is to mix waste and cement to form a hard cement solidified body after hydration, so as to reduce the leaching of hazardous components in waste (Ciotoli et al. 2020). The process of cement curing organic pollutants is actually the process of interaction between organic pollutants and inorganic cementitious materials, and its interaction includes adsorption, chemical absorption, precipitation, ion exchange, and homocrystal substitution (Chen and Wu 2018;Guo et al. 2017).
The hydration process of WDC and cement is very complex, and the hydration products will change with the content of WDC. When the solidification products of WDC and cement contact with the leaching medium, the chemical equilibrium of the solidification products is broken due to the low concentration of chemical components in the medium (Cao et al. 2020). For the hydration products formed by hydration, the first is easy to dissolve to maintain the chemical equilibrium. For Ca(OH) 2 , single sulfate, ettringite, C-S-H, and several hydration products, studies on their solubility indicate their solubility order, Ca(OH) 2 > single sulfate > AFt > ettringite > C-S-H, that is, Ca(OH) 2 is the most easy to dissolve, while ettringite and C-S-H are difficult to dissolve. From this point of view, these two hydration products play an important role in the solidification of organic pollutants (Mostavi et al. 2015;Tian et al. 2021).
Some organic pollutants in WDC mainly exist in the form of benzo(a)anthracene, benzo(k)fluoranthene, benzo(a)pyrene, and other polycyclic aromatic hydrocarbons. The presence of organic pollutants will not only interfere with the solidification and stabilization process of cement, but also change the strength of the product, making the stabilization process of pollutants more difficult (Xu et al. 2017). In general, organic compounds form a layer of organic compounds inhibiting hydration on the cement surface through ion bonds, hydrogen bonds, and dipole bonds. Through the adsorption of toxic substances by the powdery calcium silicate hydrated colloid in cement, the cementitious hydrate in cement and organic pollutants in WDC form a solid solution, so that the pollutants are bound in the cement hardening tissue, thereby greatly reducing the content of organic pollutants in WDC. The basic principle is to reduce the surface area and permeability of hazardous solidified organic polycyclic aromatic hydrocarbons by curing inclusion compounds, so as to achieve the purpose of stable and harmless organic pollutants (Ma et al. 2016;Bedia et al. 2018). The mechanism of cement solidification pollutants is shown in Fig. 8.

Treatment process of WDC
The WDC process flow chart of road cushion is shown in Fig. 9, and the distribution construction steps are as follows: Step 1 Stirring crushing and screening unit The material is sent to the mixer by shovel truck, and then sent to the roller siever for screening. The material with particle size greater than 30 mm is screened out, and small particles enter the next unit.
Step 2 Stacking unit Through the use of excavators and loaders, the screened solid waste is piled into a strip of 2 m wide and 0.5 m high.
Step 3 Flip and dosing unit The solid waste is transmitted to the dump unit by conveyor belt, and the solid waste is dumped by hydraulic-assisted Step 4 Material detection After the material treatment was completed, it is detected that the qualified material entered the material stacking area, and the unqualified material is re-entered into the stacking tank for re-stacking.

WDC detection after treatment
The treated WDC should have no obvious irritating odor, and the pollutant content should meet the requirements in Table 16; WDC are used as road cushion and well site cushion pollution control requirements in the third chapter above. In addition, in order to further ensure the environmental safety of comprehensive utilization, the oil content of WDC is set to 0.3%.
Each 300 m 3 is used as a large sampling array for pollutant content detection. Treatment of unqualified WDC: if the product is unqualified, it is necessary to re-do compound dosing treatment to meet the qualification requirements (Wang et al. 2018a, b, c, d).

Site layout
The production process of water-based drilling cuttings as road cushion is shown in Fig. 10.

Conclusions
The goal of this study is to experimentally evaluate the environmental performance and properties (including human health and ecological security risk assessment, mechanical, microstructural and environmental safety assessment) of the WDC and WDC road cushion. This study can provide a favorable way for the efficient, safe, and environmentally friendly utilization of WDC, and ensure the ecological environment safety and human health safety of WDC in resource utilization. The main results and conclusions are as follows.
The comprehensive carcinogenic risk of all exposure pathways of single pollutant benzo(a)anthracene, benzo(a)pyrene, benzo(k)fluoranthene, and indeno(1,2,3cd)pyrene was acceptable. However, the cumulative carcinogenic risk of skin contact pathway of particulate matter intake of dibenzo(a,h)anthracene was at an unacceptable level. It was considered that only dibenzo(a,h) anthracene had carcinogenic effect, so the soil risk control value of heavy metals in WDC can be calculated: the risk control limit of dibenzo(a,h)anthracene in WDC was 1.8700 mg/kg.
In this study, cement solidification was used to convert organic pollutants in WDC into harmless substances. The basic principle was that organic substances form a layer of organic substances that inhibit hydration on the cement surface through ion bonds, hydrogen bonds, and dipole bonds. Through the adsorption of toxic substances by the powdery calcium silicate hydrated colloid in cement, the cementitious hydrate in cement and organic pollutants in WDC formed a solid solution, so that the pollutants were bound in the cement hardening tissue, thereby greatly reducing the content of organic pollutants in WDC.
By studying and adjusting the formula of WDC pavement cushion, the WDC pavement cushion was finally designed by 6% cement + 50% WDC + 44% crushed stone. The 28-day unconfined compressive strength met the requirements of the Chinese standard Technical Guidelines for Construction of Highway Roadbases (JTG/T F20-2015). The environmental protection, building materials, and safety of the product were sampled and analyzed. The leaching concentration of main pollutants met the relevant standards of China.

Annex 1
In this study, the Chinese standard Technical guidelines for risk assessment of soil contamination of land for construcion (HJ 25.3-2019) is used for human health and ecological security risk assessment. Table 17 shows the calculation results of leaching dilution factor LF gw during the bedding utilization of WDC site, Table 18 shows the calculation results of groundwater drinking risk of WDC site, and Table 19 shows the calculation results of soil risk control value based on carcinogenic effect under different exposure conditions.

Annex 2
A total of WDC of three drilling platforms were leached and tested. The concentration test results are shown in Table 20. According to the standard limits of each leaching toxicity test item in the Chinese standard Identification standards for hazardous wastes-Identification for extraction toxicity (GB5085.3-2007), the pollutants in WDC were screened and the test results were analyzed. The toxic substances content of WDC as road cushion is detected, and the risk assessment of WDC as road cushion is carried out combined with the risk control limits in Soil environmental quality-Risk control standard for soil contamination of agricultural land (GB 15618-2018) and Risk screening guideline values for soil contamination of development land (three drafts) (HJ25.5-2015).
In addition, the toxicity parameters and exposure parameters of the main pollutants required for the human health evaluation of WDC as road cushion are shown in Table 21 and Table 22.