Groundwater Monitoring of Agricultural Contaminants for Regulatory Compliance

Regulatory programs frequently require monitoring of first encountered (shallow-most) groundwater for purposes of determining whether an actual or potential, permitted or incidental waste discharge has had or will have a degrading effect on groundwater quality. Traditionally, these programs have focused on monitoring of incidental discharges from industrial sites. Increasingly, sources with an implied groundwater recharge are subject to monitoring requirements. These recharging sources include, for example, land application of municipal, food processing, or animal waste to irrigated cropland. Some programs use existing production wells for ambient monitoring of agricultural contaminants. Time of travel and significant vertical mixing in the well limit the usefulness of these for source monitoring. Other programs rely on monitoring well networks similar to those on industrial sites. Groundwater monitoring of a recharging source, however, requires a different approach to groundwater monitoring than traditional (incidental source) monitoring programs. Besides being an intentional source of groundwater recharge, the scale of these applications is orders of magnitude larger than typical “point sources”. Furthermore, the shallow groundwater aquifer targeted for compliance monitoring commonly consists of highly heterogeneous unconsolidated alluvial, fluvial, lacustrine, glacial, or subaeolian sediments of late tertiary or quaternary age. Particularly in arid and semi-arid climates, groundwater is also frequently subject to significant seasonal and interannual groundwater level fluctuations that may exceed ten feet seasonally and several tens of feet within a three- to five-year period. We developed a hydrodynamically rigorous approach to designing groundwater monitoring wells for recharging sources under conditions of aquifer heterogeneity and water level fluctuations. The approach was applied to the design of a monitoring well network for monitoring confined animal farming operations (CAFOs) with irrigated crops located on alluvial fans with highly fluctuating, deep groundwater table.