Delineation of well head protection area in agricultural zone by using solute transport modelling
A wellhead protection area is the surface and subsurface area surrounding a waterwell through which contaminants are reasonably likely to move toward and reach. In the past, various approaches have been taken to delineate wellhead protection areas : fixed circles or rings around the well; simplified variable shapes based on geo-hydrologic mapping and classification ; zones with prescribed minimum travel times. However, in establishing wellhead protection areas, many factors need to be considered : zone of influence around the well; well recharge area; flow paths; transport velocities; travel times; sources and types of contamination. To determine a site-specific wellhead protection area, a systematic analytic approach must be taken. Mathematical simulation models may be employed and are often the only method capable to determine the wellhead protection area when quantitative criteria are used. Such an approach can be used in agricultural zones, where pesticides are applied, by coupling a solute transport modal for the unsaturated zone with a saturated zone transport model. The (unsaturated zone) VULPEST model is an evaluation tool for the groundwater contamination by pesticides based on the transport modeling. Developed as a management tool, it permits the evaluation of the groundwater vulnerability to pesticides in terme of risk of contamination. It evaluates the concentrations of pesticide that reach the water table, taking into account the spatial variability of hydrodynamic, physical and physicochemical parameters of the soil. The variability of parameters is taken into account in the Monte Carlo approach. This approach consists of carrying out a sufficient number of simulations so that the distribution of values assigned to each parameter, these values being randomly selected from a chosen probability distribution, approximates the given distribution. The concentrations obtained from the VULPEST model are used as input data in the model which simulates the transport and the fate of the contaminant in the saturated zone. This model uses the finite difference technique to simulate flow and solute transport. It considers the flow velocities and the influence of each well. In steady state conditions, the linked transport models in unsaturated and saturated zones may be considered independent. The quantitative results obtained by these means determine the vulnerability level of the well. Finally, they permit the delineation of the wellhead protection area for a specific contaminant, that is a given pesticide. An application was performed to an important potato crop area in Quebec. Few years ago, this site has shown a contamination of the well water by the pesticide aldicarb. The cultivated soil consists of marine and fluvial sand with medium to coarse grain sizes, deposited on a sea clay with a thickness of about 20 m in some places. Potatoes are intensively grown in this region. In the eighties, a contamination by the pesticide aldicarb was noticed in some wells of this region. The granular form of aldicarb is applied during the sowing period (mid-May) at the recommended rate of 2.24 kg/ha. It has a high solubility (6 000 mg/l) and is leached by soil humidity. The aldicarb is transformed by oxidation to sulfoxide then to sulfone during its transit in the unsaturated zone. After a characterization of the soil physical parameters, calculations were run for both the unsaturated and the saturated zones. The depths of the well and the aquifer are 5 m and 3 m respectively. The thickness of the aquifer affected by pumping is about 2 m. The application of the pesticide aldicarb was done during 1982 and 1983. The predictive results obtained by modelling for the pesticide concentrations in the well water were favorably comparerd to the concentrations measured at the site. The concentrations of pesticide in the water reach their peaks 7 weeks after every application. The maximum concentrations reaching the water table were found to be about 0,5 mg/l. This level exceeds largely the water quality criterion of 9 μg/l set by Health and Welfare Canada, and the one of 10 μg/l of the US-EPA. The well concentrations are calculated by taking into account transport in the saturated zone and decay processes. The maximum concentrations obtained are near 24 μg/l for a decay rate of the pesticide in the aquifer of 0,003 d-1. This decay rate is the one corresponding of the hall-lite of 8 months found by other researchers for Florida soils. The analysis of the water well during this period shows concentrations of about 10 μg/l. Moreover, the leaching of the contaminant into the well, and its persistence in the soil and groundwater is still present over 3 years after the last application. Using the water quality criterion of 9 μg/l set by Health and Welfare Canada, calculations have provided the delineation of the wellhead protection area specific to the pesticide aldicarb. The boundary delineation of water well protection area is determined by the numerical technique of reverse path line. The maximum extension of the well protection area obtained by this mean is 110 meters. It corresponds to a peak arrival with a decay of 1.5 years after the application. The case study shows an example of the possible and future applications for such a method for the delineation of the wellhead protection areas. Such an approach permits to council the best use of pesticides with an appropriate groundwater protection scheme, indeed, agricultural managers can safely decide on the pesticide application rate and date, as on the choice between various pesticides, with regard to the groundwater quality protection. Through this way, regulators and scientists can base their decisions for the registration of new pesticides by testing, before their use, their possible impacts on groundwater. Comparisons can be easily doge between water quality criteria and predicted quantifies, and regulatory decisions can be taken in light of these results.