Sources of high-chloride water and managed aquifer recharge in an alluvial aquifer in California, USA

Abstract

As a result of pumping in excess of recharge, water levels in alluvial aquifers within the Eastern San Joaquin Groundwater Subbasin, 130 km east of San Francisco (California, USA), declined below sea level in the early 1950s and have remained so to the present. Chloride concentrations in some wells increased during that time and exceeded the US Environmental Protection Agency's secondary maximum contaminant level of 250 mg/L, resulting in removal of some wells from service. Sources of high-chloride water include irrigation return in 16 % of sampled wells and water from delta sediments and deeper groundwater in 50 % of sampled wells. Chloride concentrations resulting from irrigation return commonly did not exceed 100 mg/L, although nitrate concentrations were as high as 25 mg/L as nitrogen. Chloride concentrations ranged from less than 100-2,050 mg/L in wells affected by water from delta sediments and deeper groundwater. Sequential electromagnetic logs show movement of highchloride water from delta sediments to pumping wells through permeable interconnected aquifer layers. δD and δ 18O data show most groundwater originated as recharge along the front of the Sierra Nevada, but tritium and carbon-14 data suggest recharge rates in this area are low and have decreased over recent geologic time. Managed aquifer recharge at two sites show differences in water-level responses to recharge and in the physical movement of recharged water with depth related to subsurface geology. Well-bore flow logs also show rapid movement of water from recharge sites through permeable interconnected aquifer layers to pumping wells.

Depth‐Resolved Groundwater Chemistry by Longitudinal Sampling of Ambient and Pumped Flows Within Long‐Screened and Open Borehole Wells

“…Depth-specific samples and measurements of vertical flow rate before and after inflowing zones are combined in a mass balance to identify the concentration of inflowing groundwater. Collar and Mock (1997) appear to be the first to report this method, but it was subsequently adopted by the U.S. Geological Survey (Izbicki et al, 1999;Izbicki, 2004) and used to identify water quality issues (arsenic and salinity) in operational supply wells (Goldrath et al, 2015;Halford et al, 2010;Izbicki et al, 2005Izbicki et al, , 2006Izbicki et al, , 2008Izbicki et al, , 2010Jurgens et al, 2014;Landon et al, 2010;O'Leary et al, 2012O'Leary et al, , 2015Smith, 2005). Where such wells are regularly pumped at a high rate, the effect of ambient flows will be minimised, and inflows at all depths are likely to be groundwater native to the source interval.…”

Chemical and Microbial Markers for Discriminating Sanitary Sewer Contamination in Coastal, Urban Streams

“…Rainwater is generally very dilute, and mean annual chloride concentrations of wet deposition for National Atmospheric Deposition Program sites in California (CA42 and CA67) and 2018 chloride concentrations for precipitation collected in San Diego are both <2.0 mg/L (Figure a). By contrast, groundwater in some parts of southern California generally has higher chloride concentrations due to its location near the coast and other subsurface geologic characteristics. , In San Diego, the exploitation of groundwater aquifers in the 1900s resulted in seawater intrusion causing high groundwater chloride concentrations . Our study shows that the Cl/Br signature of a range of groundwater wells in San Diego, at values >300, is similar to the Cl/Br mass ratio of the San Diego River at baseflow (Figure a).…”

“…By contrast, groundwater in some parts of southern California generally has higher chloride concentrations due to its location near the coast and other subsurface geologic characteristics. 51,52 In San Diego, the exploitation of groundwater aquifers in the 1900s resulted in seawater intrusion causing high groundwater chloride concentrations. 53 Our study shows that the Cl/Br signature of a range of groundwater wells in San Diego, at values >300, is similar to the Cl/Br mass ratio of the San Diego River at baseflow (Figure 1a).…”

Numerical Study on the Effects of Regional Groundwater Flow on the Contaminant Migration through an Inactive Supply Well as a Preferential Flow Pathway

“…To quantitatively evaluate the influences of the regional groundwater velocity on the deterioration of water quality, the recovery ratio of contaminant was computed at the end of each pumping phase with different regional groundwater velocities. Table 2 represent the recovery ratio based on Equation (19) with different given values of regional velocities when t pump = 90 and 180 days, respectively. It is apparent that a higher flow velocity leads to a smaller recovery ratio in the cases of t pump = 90 and 180 days, which means a higher flow velocity results in a more serious water deterioration in the deep aquifer.…”

“…It takes only a short time for these contaminants to penetrate sediments facilitated by ISWs, resulting in significant contamination of deep aquifers [11,17]. On the other hand, if the hydraulic head in deep aquifer is higher than in shallow aquifer, environmentally hazardous substances like arsenic, chloride, or hexavalent chromium in deep aquifers can be carried upwards into shallow aquifers [18,19].…”