Identifying areas of degrading and improving groundwater-quality conditions in the State of California, USA, 1974–2014

Abstract

Areas of improving and degrading groundwater-quality conditions in the State of California were assessed using spatial weighting of a new metric for scoring wells based on constituent concentrations and the direction and magnitude of a trend slope (Sen). Individual well scores were aggregated across 2135 equal-area grid cells covering the entire groundwater resource used for public supply in the state. Spatial weighting allows results to be aggregated locally (well or grid cell), regionally (groundwater basin), provincially, or statewide. Results differentiate degrading (increasing concentration trends) areas with low to moderate concentrations (unimpaired) from degrading areas with moderate to high concentrations (impaired). Results also differentiate improving areas (decreasing concentration trends) in the same manner. Multi-year to decadal groundwater-quality trends were computed from periodic, inorganic water-quality data for 38 constituents collected between 1974 and 2014 for compliance monitoring of nearly 13,000 public-supply wells (PSWs) in the State of California. Mann-Kendall (MK) rank correlations and Sen's slope estimator were used to detect statistically significant trends for the entire period of recorded data (long-term trend), for the period since 2000 (recent trend), for different pumping seasons (seasonal trend), and for reversals of trends. Statewide, the most frequently detected trends since 2000 were for nitrate (36%), gross alpha/uranium (10%), arsenic (14%), total dissolved solids (TDS) (23%), and the major ions that contribute to TDS (19-28%). The Transverse and Selected Peninsular Ranges (TSPR) and the San Joaquin Valley (SJV) hydrogeologic provinces had the largest percentage of areas with moderate to high nitrate concentrations and groundwater quality trends. Improving nitrate concentrations in parts of the TSPR is associated with long-term managed aquifer recharge that has replaced historical, agriculturally affected groundwater with low-nitrate recharge in parts of the TSPR. This example suggests that application of dilute, excess surface water to agricultural fields during the winter could improve groundwater-quality in the SJV over the long term.

Critical Aquifer Overdraft Accelerates Degradation of Groundwater Quality in California's Central Valley During Drought

“…We used nitrate as a proxy for overall groundwater quality in the Central Valley because it is required by law to be sampled from active PSWs on an annual basis and often co-occurs with a host of other contaminants that are monitored with less frequency (e.g., fumigants, salinity, and uranium; Burow et al, 2019;Rosen et al, 2019). To assess ambient conditions in aquifers used for public drinking-water supply, we compiled nitrate measurements for over 160,000 raw (untreated and unblended) groundwater samples collected from PSWs for regulatory-compliance monitoring (SWRCB-DDW, 2019a) and the California State Water Board's Groundwater Ambient Monitoring and Assessment (GAMA) program (Jurgens et al, 2018). Nitrate measurements for individual wells were seasonally averaged by "water-level year" to ensure wells with higher sampling frequency were not overrepresented in our analysis (Text S2).…”

“…The total area of increasing nitrate was >50% for most of the period of study, reflecting widespread trends of degrading groundwater quality in the Central Valley (Burow et al, 2013;Jurgens et al, 2020), and expanded up to 65% during drought (Figure 3a). However, the area of increasing nitrate contracted to <50% during brief recovery periods following drought peaks in 2004-2017.…”

“…done to connect groundwater-level decline to observed trends of degrading groundwater quality related to legacy agricultural land use (Burow et al, 2013;Jurgens et al, 2020). Approximately 3 million residents of the Central Valley depend on public-supply wells (PSWs) for drinking water (Belitz et al, 2015), which are clustered around major urban centers within the largely agricultural landscape (Figure 1b).…”

Assessment of Shallow Groundwater Purification Processes after the Construction of a Municipal Sewerage Network

“…The other dominant cluster contained northern wells in 74%. In addition, a third cluster was also formed the five wells of which can be found in the central part of the settlement (17,18,24,25,27). In the fifth year following the establishment of the sewerage network, this clear difference between wells was eliminated with a decrease in the level of contamination, with approximately the same proportion (52-48%) of north and south wells in the two established clusters.…”

“…Preserving and improving groundwater quality in the urban environment is a complex task due to the many sources of pollution [23][24][25]. Although groundwater monitoring is of great interest to water decision-makers for establishing strategies to protect water resources, due to the high costs there is not enough data available at a sufficient level of detail, especially for long-term monitoring [26,27].…”