Trend Monitoring Networks--GAMA-PBP Program Evolution
"Temporal trends in groundwater quality cannot be detected in individual wells by comparing results from just two samples collected only three years apart" (Kent and Landon, 2016).
The First Network of Public Supply Wells
The GAMA-PBP’s initial goal was to provide information on statewide groundwater-quality trends by re-sampling 10% of the public supply wells used in individual study areas every three years, and to re-sample the whole network of public supply wells every ten years (Belitz and others, 2003). This network design is consistent with the USGS National Water Quality Assessment trends characterization system (Lindsey and Rupert, 2012) and supports identification of step-trends over broad areas.
The first GAMA-PBP trend analysis was carried out by Kent and Landon (2016), who identified 3-year step trends for 17 constituents in five different hydrogeologic zones.
Adding the Domestic Well Network
As the first statewide assessment was coming to a close, resource managers across the State indicated a need to understand the part of the aquifer system suppling domestic users in the same level of detail as the initial program targeting the part of the aquifer supplying public drinking-water systems. This need was driven by clear differences observed in water quality from wells at different depths in aquifers, an understanding that downward migration of water from the shallower zones where most domestic wells are would eventually mix into the deeper zones where most public supply wells are, and a new emphasis by the State on the human right to clean water and a need to identify where domestic well users might need to shift to treated water supplies. In response, the GAMA-PBP began developing a second statewide network of wells focused on groundwater resources tapped for domestic supply, and cut back on trend monitoring in the public supply aquifer network.
Because of the programmatic expansion cost to include domestic wells, and for logistical reasons including closure of wells in the trend network, the approach shifted to re-visiting approximately 20% of the public supply and domestic well networks every 5-10 years. Calculating step trends with these data requires grouping samples over larger geographic areas--as a result, GAMA-PBP trends will be reported for spatial areas larger than the initial study units.
Kent (2018) improved the statistical methods for identifying trends in GAMA-PBP data by creating a method that accounts for variability expected between replicate samples. This proof of concept documented decreasing arsenic concentrations over a ten-year period in North San Francisco Bay and increasing concentrations of nitrate in Monterey Bay/ Salinas Valley study areas.
Additional efficiencies in trend monitoring and appropriate time and spatial scales for GAMA-PBP focus are currently being explored.
The GAMA-PBP trend sampling data from both the public and domestic well networks can be viewed and downloaded from this map interface.
The GAMA-PBP also conducts detailed interpretive studies of trends in groundwater quality which use data beyond the samples collected by the GAMA-PBP monitoring program.
Some of the questions these studies seek to answer include
- How will the groundwater quality in the shallower parts of the aquifer affect resources used by public supply wells? Over what periods? Under what types of pumping and flow conditions?
- How fast is groundwater quality changing in the aquifer supplying public wells? Is that change different across whole basins compared to individual wells?
- Are there basin-scale trends in groundwater-quality changes and what management activities, including contaminant loading reduction, pumping, and recharge are linked to those trends?
In carrying out these trend studies, the GAMA-PBP carefully considers local trend-monitoring programs and data, including sampling carried out including the Irrigated Lands Regulatory Program, CV-Salts Surveillance and Monitoring Program, and evolving groundwater-quality monitoring conducted by Groundwater Sustainability Agencies.