Real-Time Monitoring to Assess Temporal Changes in Groundwater Quality

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 two wells in the San Joaquin Valley California, USA. The water-quality parameters pH, temperature (T), dissolved oxygen (DO), and specific conductance (SC) are measured (15 minute intervals) at continuously pumping supply wells (190 m and 98 m depths), and the data are transmitted by GOES satellite.

Groundwater is sampled for nitrate (NO₃), DBCP (Dibromochloropropane), perchlorate, ²²²Rn, δD, δ¹⁸O, and major ions bimonthly at the wells in an effort to correlate changes in pH, T, DO, and SC with these constituents of interest. NO₃, DBCP, ²²²Rn, and perchlorate were detected above maximum contaminant levels or at elevated concentrations.

The first year of data (2013-2014) show mostly constant groundwater quality, including the constituents of interest for both wells. Real-time data, however, also provide information about the sensitivity of the data to external factors. These include interruption in pump operation (turbine pump on and off) due to power disruption or periodic maintenance, which caused notable variability in the DO and SC data. Sonde mounting orientation also affected the data due to bubble accumulation on the probes. These factors must be considered to ensure a robust and accurate temporal record.