The layer shows wells identified by USGS as having low water levels.
"A well with below normal groundwater levels is identified on these pages when the most recent water-level measurement is in the 24th percentile or lower in the month of measurement over the period of record for the well. To be included on this map in red or orange, the well must be in an active measurement program (the well appears on the Active Groundwater Level Network) and the well must have 10 or more years of record in the month of the most recent measurement." (http://groundwaterwatch.usgs.gov/Net/OGWNetwork.asp?ncd=lwl)
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