Overview of Water Quality

GAMA’s Priority Basin Project evaluates the quality of untreated groundwater. However, for context, benchmarks established for drinking-water quality are used for comparison. Benchmarks and definitions of high, moderate, and low concentrations are discussed in the inset box on page 3.

Many inorganic constituents occur naturally in groundwater. The concentrations of the inorganic constituents can be affected by natural processes as well as by human activities. In the alluvial-filled basins of the San Diego study unit, one or more inorganic constituents were present at high concentrations in 18 percent of the primary aquifers and at moderate concentrations in 32 percent of the primary aquifers.

Organic constituents are in products used in the home, business, industry, and agriculture. Organic constituents can enter the environment through normal usage, spills, or improper disposal. In the alluvium-filled basins of the study unit, one or more organic constituents were present at high concentrations in 3 percent of the primary aquifers and at moderate concentrations in 3 percent of the primary aquifers.

RESULTS: Groundwater Quality in the San Diego Study Unit

Inorganic Constituents with Human-Health Benchmarks

Trace and minor elements are naturally present in the minerals in rocks and soils, and in the water that comes into contact with those materials. In the alluvial-filled basins in the San Diego study unit (Temecula Valley, Warner Valley, and Alluvial Basins), one or more trace elements were present at high concentrations in about 14 percent of the primary aquifers. Vanadium, arsenic, and boron were detected at high concentrations in at least 4 percent of the primary aquifers. Antimony was detected at high concenetrations in about 3 percent of the primary aquifers.

Radioactivity is the release of energy or energetic particles during structural changes in the nucleus of an atom. Most of the radioactivity in groundwater comes from decay of naturally occurring isotopes of uranium and thorium in minerals in the sediments of the aquifer. Radioactivity from the radioactive constituents occurred at high concentrations in 3 percent of the primary aquifers, and at moderate concentrations in 14 percent of the primary aquifers. Gross-alpha radioactivity was detected at high concentrations in 3 percent of the primary aquifers.

Nutrients, such as nitrate and nitrite, are naturally present at low concentrations in groundwater. High and moderate concentrations generally occur as a result of human activities, such as applying fertilizer to crops. Septic systems and livestock, when in concentrated numbers, also produce nitrogenous waste that can leach into groundwater. Nutrients, in the form of nitrate, were present at high concentrations in 3 percent of the primary aquifers, and at moderate concentrations in about 7 percent of the primary aquifers.

Inorganic Constituents with Non-Health Benchmarks

(Not included in water-quality overview charts shown on the front page)

Some constituents, such as total dissolved solids (TDS), affect the aesthetic properties of water, such as taste, color, and odor. Other constituents, such as iron and manganese, can create nuisance problems, such as scaling and staining.

The State of California has a recommended an upper limit for TDS in drinking water. In the alluvial-fill basins of the San Diego study unit, TDS was present at high concentrations (greater than the upper limit) in 14 percent of the primary aquifers, at moderate concentrations (between the recommended and upper limit) in 31 percent of the primary aquifers, and at low concentrations (less than the recommended limit) in 55 percent of the primary aquifers.

Iron and manganese are naturally occurring elements, and one or both were present at high concentrations in 14 percent of the primary aquifers. Iron or manganese (or both) were present at moderate concentrations in 3 percent of the primary aquifers, and at low concentrations in about 83 percent of the primary aquifers.

Perchlorate, 1,4-dioxane, and N-Nitrosodimethylamine (NDMA)

(Not included in water-quality overview charts shown on the front page)

Perchlorate, 1,4-dioxane, and NDMA are of special interest in California because these constituents recently have been detected in groundwater and are considered to have the potential to affect the quality of drinking-water supplies. Their presence in groundwater is monitored by the California Department of Public Health (http://www.cdph.ca.gov). In the San Diego study unit, perchlorate was detected at high concentrations in less than 1 percent of the primary aquifers and at moderate concentrations in about 36 percent of the primary aquifers. 1,4-dioxane and NDMA were not detected.

Organic Constituents

The Priority Basin Project uses laboratory methods that can detect low concentrations of volatile organic compounds (VOCs) and pesticides, far below human-health benchmarks. The presence of VOCs and pesticides detected at these low concentrations can be used to trace water from the landscape into the aquifer system.

Volatile Organic Compounds with Human-Health Benchmarks

VOCs are in many household, commercial, industrial, and agricultural products and are characterized by their tendency to volatilize into the air.

Gasoline components are compounds that constitute gasoline, or are additives, such as methyl tert-butyl ether (MTBE). In the alluvium-filled basins of the San Diego study unit, gasoline components were present at high concentrations in 3 percent of the primary aquifers. The gasoline component detected at high concentration was MTBE. No gasoline components were detected at moderate concentrations.

Solvents are used for a number of purposes, including manufacturing and cleaning. Solvents were not detected at high concentrations, but were detected at moderate concentrations in 3 percent of the primary aquifers. The solvent detected at moderate concentrations was 1,2-dichloropropane.

Trihalomethanes may form during municipal water purification and enter groundwater by the infiltration of landscape irrigation water. Trihalomethanes were detected only at low concentrations in the primary aquifers.

Other VOCs, including organic synthesis reagents and refrigerants, were not detected either at high or at moderate concentrations.

Pesticides with Human-Health Benchmarks

Pesticides are applied to crops, gardens, lawns, around buildings, and along roads to help control unwanted vegetation (weeds), insects, fungi, and other pests. In the San Diego study unit, all pesticides, which include herbicides, insecticides, and fumigants, were detected only at low concentrations.

Factors that Affect Groundwater Quality

The trace elements vanadium (V) and boron (B) were detected at high and moderate concentrations in the San Diego study unit (Wright and Belitz, 2011). High concentrations for these constituents were detected almost exclusively in samples collected in the Temecula Valley study area.

Natural sources of these constituents to groundwater primarily are from the dissolution of rocks. Vanadium is most abundant in mafic rocks, whereas B tends to be most abundant in shale and schist (Reimann and de Caritat, 1998). Potential anthropogenic sources of V to the environment include the burning of residual crude oil, and the waste streams associated with the ferrous metallurgy industry. Boron is found in cleaning agents and therefore may be present in municipal and industrial wastes. The human-health non-regulatory bench marks (notification levels, NL) for V and B are 50 and 1,000 micrograms per liter, respectively.

The mobility of V and B in groundwater is controlled by the pH and (or) reduction-oxidation (redox) conditions of the aquifer system. Redox conditions are used to describe whether groundwater has oxidizing (oxic) or reducing (anoxic) conditions.

Vanadium exists in multiple oxidation states in the environment (Hem, 1985); therefore, its molecular structure is affected by both redox and pH. The positive correlation of V concentrations from samples collected in the San Diego study unit to oxic and alkaline (high pH) groundwater conditions was statistically significant (Wright and Belitz, 2011).

Boron does not exist in multiple oxidation states in the environment; therefore, its molecular structure is affected by pH, but not by redox conditions (Hem, 1985). The positive correlation of B concentrations from samples collected in the San Diego study unit to alkaline groundwater conditions was statistically significant, but the correlation to redox conditions was not significant (Wright and Belitz, 2011).

Priority Basin Assessments

GAMA’s Priority Basin Project (PBP) assesses water quality in that part of the aquifer system used for drinking water, primarily public supply. Water quality in shallow and deep parts may differ from water quality in the primary aquifers. GAMA’s Domestic Well Project assesses water quality in the shallow parts of the aquifer system. Ongoing assessments are being conducted in more than 120 basins throughout California.

The PBP assessments are based on a comparison of constituent concentrations in untreated groundwater with benchmarks established for the protection of human health and for aesthetic concerns. The PBP does not evaluate the quality of drinking water delivered to consumers.

The PBP uses two scientific approaches for assessing groundwater quality. The first approach uses a network of wells to statistically assess the status of groundwater quality. The second approach combines water-quality, hydrologic, geographic, and other data for use in the assessment of the factors that affect water quality. In the San Diego study unit, data were collected by the PBP in 2004 and from the CDPH database for 2001–2004. The PBP includes chemical analyses generally not available as part of regulatory compliance monitoring, including measurements at concentrations much lower than human-health benchmarks, and measurement of constituents that can be used to trace the sources and movement of groundwater.

First posted December 20, 2011