NITRATE CONCENTRATIONS IN GROUNDWATER OF THE SAN MATEO MIDCOAST, CALIFORNIA, A HISTORIC COMPARISON

The San Mateo Midcoast groundwater basin is located on the northwestern flank of the Santa Cruz Mountains in central San Mateo County. It is the main groundwater source for residential and agricultural use. Due to increased urban development in the region, residential neighborhoods have been placed adjacent to nurseries and other agricultural fields that have existed for well over a century. In recent years concerns over the aquifers sustainability and potential impact of nitrate contamination in domestic wells have risen. Previous age dating of well water in the area was performed by Lawrence Livermore National Lab and suggested a recharge rate of roughly four years for the basin. In this study, we resampled approximately 50 residential wells for nitrate contamination and compared them to results from fifteen to twenty years ago. In both instances the majority of wells exceeded maximum contaminant levels (MCL) set by the US Environmental Protection Agency (EPA). In addition, the spatial distribution of nitrate concentrations varied between the hydrostragraphic units. Given the rapid movement of groundwater, if pollution sources are properly identified and mitigation measures put in place, then nitrate levels might be brought down below maximum concentration levels (MCL).