A Wellhead Protection Study Of A Small Agricultural Community In Glaciated Terrain, As A Model For Wellhead Protection Efforts By Similar Communities
The purpose of this study was to create a wellhead protection approach for a small town in a rural watershed that could be used by other, similar, communities.
Lacustrine clay and melt-out till interbedded in outwash sands and fine gravel, separate an unconfined aquifer from a deeper aquifer. The lower aquifer was determined to be semiconfined due to (a) high vertical conductivity, (b) lack of continuity in the clay/till, (c) absence of significant seasonal water level changes, and (d) tritium levels above 1-2 tritium units in deep wells.
Tritium level falls and water age rises along flow-paths. A low-high-low pattern of tritium was observed with depth, likely representing the peak years of nuclear weapons testing. Samples of ground water discharging from wetlands more enriched by about 4 per mil in the stable oxygen isotope, plot below the meteoric water line with a reduced slope and suggest evaporative enrichment. The stable nitrogen isotope was useful in determining the source of nitrate contamination.
Although all samples were below the MCL for atrazine, the highest levels were near corn fields or farm wells. Wetlands may retard atrazine transport.
Seasonal change in hydrogeochemistry was observed between the fall and spring in wells and in ground water discharging from wetlands. Sites of water quality degradation by agriculture and road salt were characterized by elevated levels of some or all of: alkalinity, calcium, chloride, nitrate, potassium, and sodium. An increase was observed in alkalinity and a decrease was seen in calcium, chloride, nitrate, potassium, sulfate, and atrazine along the north-south flow-paths from recharge to discharge areas.
MODFLOW, WHPA, and WhAEM, the new analytic element model, yielded travel times similar to that from tritium. Domestic well logs provided reliable lithologic data and calibration in excellent agreement with dedicated monitoring wells.
It is recommended that a rural village begin wellhead protection with a simple model utilizing available data from domestic well logs and topographic maps. Additional data and more comprehensive models should be used only as need or geological complexity demand.