Source and Transport Controls on the Movement of Nitrate to Public Supply Wells in Selected Principal Aquifers of the United States

Understanding factors controlling the source and transport of nitrate (NO3-) to public-supply wells (PSW) is challenging because of complex areas contributing recharge to the wells, spatial and temporal variability in NO3- sources within the contributing areas, spatially variable denitrification rates along flow paths leading to the wells, mixing, and possible perturbations in the flow system caused by well construction and (or) operation. Systematic studies of NO3- movement to PSW in four diverse hydrogeologic environments showed that NO3- in PSW was derived from varying proportions of agricultural, urban, and natural sources. Nitrate fluxes to the water table were larger in agricultural settings than in urban and natural settings, indicating the PSW capture zones should be designed to limit inputs from agricultural sources. Denitrification in the aquifers was characterized by either slow rates (≤0.02/yr) in broad reaction zones within sandy fluvial and alluvial-fan deposits or by fast rates (0.1 to 6/yr) in thin reaction zones at clay/sand contacts of glacial and marine deposits. In undisturbed flow systems, denitrification in the thin reaction zones provided more protection against NO3- contamination than the broad reaction zones. However, well construction and (or) operational features caused high NO3- water in two study areas to partially bypass or move more quickly through the denitrifying zones. In one sand aquifer, shallow NO3- contaminated water bypassed the thin reaction zone and entered the producing interval of the deeper PSW by moving down long well screens of nearby irrigation wells. In one carbonate aquifer, shallow NO3- contaminated water bypassed the thin reaction zone and entered the producing interval of the PSW by moving quickly through a high-permeability karst conduit. Consideration should be given to constructing and managing PSW to minimize bypass of denitrifying zones to take full advantage of the natural NO3- attenuation capacity of aquifer sediments.