34 research outputs found
Identifying the location and population served by domestic wells in California
Study region: California, USA. Study focus: Identification of groundwater use is an important step in the regional-scale assessment of groundwater quality. In California, 1990 US Census data indicate that domestic wells provide drinking-water to about 1.2 million people. However, the location of these domestic well users of groundwater is poorly identified because the census tracts can be quite large (up to 20,000Â km2). The purposes of this paper are to present methods used for (1) estimating the location of domestic wells, (2) estimating the location of households using domestic well water; and (3) identifying where in California groundwater is an important source of domestic drinking supply. New hydrological insights for the region: Aggregating the results indicates that three hydrogeologic provinces contain nearly 80% of all domestic wells and also have the highest density of domestic well users: Central Valley (31.6%), Sierra Nevada (31.5%), and Northern Coast Ranges (16.6%). Results were also aggregated into groundwater basins and highland areas, collectively called Groundwater Units (GUs). Twenty-eight of the 938 GUs contain more than 50% of the total population served by domestic wells, 70 GUs contain more than 75%, and 150 GUs contain 90%. The 28 GUs are mostly located in the eastern and southern San Joaquin Valley (11), the Sacramento Valley (7), and the western foothills of the Sierra Nevada province (5). Using the information presented in this research along with other information about domestic-well use, the US Geological Survey has begun sampling high-use GUs for the Shallow Aquifer Assessment component of the Groundwater Ambient Assessment (GAMA) program. Keywords: Domestic wells, Drinking water, Groundwater, Statistical sampling, Hydrogeologic province, Californi
Effects of Groundwater Development on Uranium: Central Valley, California, USA
Uranium (U) concentrations in groundwater in several parts of the eastern San Joaquin Valley, California, have exceeded federal and state drinking water standards during the last 20 years. The San Joaquin Valley is located within the Central Valley of California and is one of the most productive agricultural areas in the world. Increased irrigation and pumping associated with agricultural and urban development during the last 100 years have changed the chemistry and magnitude of groundwater recharge, and increased the rate of downward groundwater movement. Strong correlations between U and bicarbonate suggest that U is leached from shallow sediments by high bicarbonate water, consistent with findings of previous work in Modesto, California. Summer irrigation of crops in agricultural areas and, to lesser extent, of landscape plants and grasses in urban areas, has increased Pco2 concentrations in the soil zone and caused higher temperature and salinity of groundwater recharge. Coupled with groundwater pumping, this process, as evidenced by increasing bicarbonate concentrations in groundwater over the last 100 years, has caused shallow, young groundwater with high U concentrations to migrate to deeper parts of the groundwater system that are tapped by public-supply wells. Continued downward migration of U-affected groundwater and expansion of urban centers into agricultural areas will likely be associated with increased U concentrations in public-supply wells. The results from this study illustrate the potential long-term effects of groundwater development and irrigation-supported agriculture on water quality in arid and semiarid regions around the world
Geogenic Sources of Benzene in Aquifers Used for Public Supply, California
Statistical evaluation of two large statewide data sets
from the
California State Water Board’s Groundwater Ambient Monitoring
and Assessment Program (1973 wells) and the California Department
of Public Health (12 417 wells) reveals that benzene occurs
infrequently (1.7%) and at generally low concentrations (median detected
concentration of 0.024 μg/L) in groundwater used for public
supply in California. When detected, benzene is more often related
to geogenic (45% of detections) than anthropogenic sources (27% of
detections). Similar relations are evident for the sum of 17 hydrocarbons
analyzed. Benzene occurs most frequently and at the highest concentrations
in old, brackish, and reducing groundwater; the detection frequency
was 13.0% in groundwater with tritium <1 pCi/L, specific conductance
>1600 μS/cm, and anoxic conditions. This groundwater is typically
deep (>180 m). Benzene occurs somewhat less frequently in recent,
shallow, and reducing groundwater; the detection frequency was 2.6%
in groundwater with tritium ≥1 pCi/L, depth <30 m, and anoxic
conditions. Evidence for geogenic sources of benzene include: higher
concentrations and detection frequencies with increasing well depth,
groundwater age, and proximity to oil and gas fields; and higher salinity
and lower chloride/iodide ratios in old groundwater with detections
of benzene, consistent with interactions with oil-field brines
Metrics for Assessing the Quality of Groundwater Used for Public Supply, CA, USA: Equivalent-Population and Area
Data from 11 000
public supply wells in 87 study areas were
used to assess the quality of nearly all of the groundwater used for
public supply in California. Two metrics were developed for quantifying
groundwater quality: area with high concentrations (km<sup>2</sup> or proportion) and equivalent-population relying upon groundwater
with high concentrations (number of people or proportion). Concentrations
are considered high if they are above a human-health benchmark. When
expressed as proportions, the metrics are area-weighted and population-weighted
detection frequencies. On a statewide-scale, about 20% of the groundwater
used for public supply has high concentrations for one or more constituents
(23% by area and 18% by equivalent-population). On the basis of both
area and equivalent-population, trace elements are more prevalent
at high concentrations than either nitrate or organic compounds at
the statewide-scale, in eight of nine hydrogeologic provinces, and
in about three-quarters of the study areas. At a statewide-scale,
nitrate is more prevalent than organic compounds based on area, but
not on the basis of equivalent-population. The approach developed
for this paper, unlike many studies, recognizes the importance of
appropriately weighting information when changing scales, and is broadly
applicable to other areas
Metrics for Assessing the Quality of Groundwater Used for Public Supply, CA, USA: Equivalent-Population and Area
Data from 11 000
public supply wells in 87 study areas were
used to assess the quality of nearly all of the groundwater used for
public supply in California. Two metrics were developed for quantifying
groundwater quality: area with high concentrations (km<sup>2</sup> or proportion) and equivalent-population relying upon groundwater
with high concentrations (number of people or proportion). Concentrations
are considered high if they are above a human-health benchmark. When
expressed as proportions, the metrics are area-weighted and population-weighted
detection frequencies. On a statewide-scale, about 20% of the groundwater
used for public supply has high concentrations for one or more constituents
(23% by area and 18% by equivalent-population). On the basis of both
area and equivalent-population, trace elements are more prevalent
at high concentrations than either nitrate or organic compounds at
the statewide-scale, in eight of nine hydrogeologic provinces, and
in about three-quarters of the study areas. At a statewide-scale,
nitrate is more prevalent than organic compounds based on area, but
not on the basis of equivalent-population. The approach developed
for this paper, unlike many studies, recognizes the importance of
appropriately weighting information when changing scales, and is broadly
applicable to other areas