Assessing Variability in Groundwater Quality in the San Joaquin Valley, California with Continuous Monitoring

In an effort to better understand how groundwater quality changes over short (daily to monthly) and long (seasonal to decadal) timescales, the USGS NAWQA Program collects continuous (high frequency) data on water quality in near real time at three wells in the San Joaquin Valley, California, USA. The water-quality parameters pH, temperature (T), dissolved oxygen (DO), and specific conductance (SC) are measured (30-minute intervals) at continuously pumping wells (190 m and 98 m depths), and the data are transmitted hourly to the National Water Information System online database for near real time viewing. Nitrate (NO₃) also is measured optically at the deep well and the data transmitted. A monitoring well (71 m depth) is instrumented to pump once per day, measure T, DO, pH, and SC, and transmit the data.

Groundwater also is sampled bimonthly at each well for nitrate (NO₃), DBCP (Dibromochloropropane), perchlorate, δD, δ¹⁸O, ³H, ¹⁴C and major ion in an effort to correlate changes in pH, T, DO, SC, and NO₃ with these constituents of interest. NO₃, DBCP, and perchlorate were detected above maximum contaminant levels or at elevated concentrations in the shallower two wells.

The first years of data (2013-2015) show mostly constant groundwater quality in the deep well (190 m), which suggest that the deep aquifer is isolated from surface activities by low groundwater flow. This agrees with non-detectable ³H and low ¹⁴C activities in the deep groundwater. However, the shallow wells (98 m and 71 m) show increasing SC, NO₃, and uranium levels, and have high ³H and ¹⁴C activities, suggesting more rapid communication with recharge at the land surface.