Land use in the study unit is approximately 59 percent (%) agricultural, 34% natural (primarily grassland), and 7% urban. The primary crops are almonds, walnuts, peaches, grapes, grain, corn, and alfalfa. The largest urban areas (2003 population in parentheses) are the cities of Modesto (206,872), Turlock (63,467), and Merced (69,512).

Municipal water use accounts for about 5% of the total water use in the Central Eastside study unit, with the remainder used for irrigated agriculture. Groundwater accounts for about 75% of the municipal supply, and surface water accounts for about 25%. Recharge to the groundwater flow system is primarily from percolation of irrigation return, precipitation, seepage from reservoirs and rivers, and urban return (Burow and others, 2004; Phillips and others, 2007). The primary sources of discharge are pumping for irrigation and municipal supply, evaporation from areas with a shallow depth to water, and discharge to streams. Recharge at shallow depths and pumping from wells at greater depths causes downward movement of groundwater in the aquifer in the Central Eastside. This vertical movement of water has the potential to carry chemical constituents from shallow depths to the greater depths where supply wells commonly are perforated.

Inorganic constituents with human-health benchmarks

Trace elements are naturally present in rocks, soils and minerals, and in the water that comes into contact with those materials. In the Central Eastside study unit, trace elements are present at high concentrations in about 17% of the primary aquifer. Arsenic and vanadium are the two trace elements that most frequently occur at concentrations above benchmarks. Trace elements are present at either low or moderate concentrations in about 83% of the primary aquifer.

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 that are present in minerals in the sediments of the aquifer. Radiologic constituents occur at values above benchmarks in about 4% of the primary aquifer and at moderate values in about 12%. Radiologic constituents are present at low values in about 84% of the primary aquifer.

Nutrients, such as nitrate, are naturally present at low concentrations in groundwater. High and moderate concentrations generally occur owing to human factors such as fertilizer application or septic-system seepage. Nitrate is present at high concentrations in about 2% of the primary aquifer. Nutrients are present at moderate concentrations in about 15% of the primary aquifer. Concentrations of nutrients are low in 83% of the primary aquifer in the Central Eastside study unit.

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, or odor. Other constituents, such as iron and manganese, can create nuisance problems, such as scaling and staining.

The State of California has a recommended and an upper limit for TDS in drinking water. In the Central Eastside, TDS is present at high concentrations (greater than the upper limit) in about 2% of the primary aquifer. About 9% of the primary aquifer has moderate TDS concentrations (between the recommended and upper limit), and about 89% has low concentrations (less than the recommended limit).

Iron and manganese are naturally occurring elements and are present at high concentrations in about 5% of the primary aquifer. Iron or manganese are present at moderate concentrations in about 9% of the primary aquifer, and are low in about 86%.

Perchlorate

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

Perchlorate is an inorganic constituent of special interest in California because it has recently been found in, or is considered to have the potential to reach, drinking-water supplies, and its presence in groundwater is monitored by the California Department of Public Health (http://www.cdph.ca.gov). Perchlorate is an ingredient in rocket fuel, fireworks, safety flares, and other products, but can also be present in some fertilizers. Perchlorate can also be naturally present in areas with an arid climate.

In the Central Eastside, perchlorate was not detected at high concentrations in the primary aquifer. Perchlorate was detected at moderate concentrations in about 21% of the primary aquifer and was low or not detected in about 79%.

Organic Constituents

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

Volatile organic compounds with human-health benchmarks

Volatile organic compounds (VOCs) are present in many household, commercial, industrial, and agricultural products, and are characterized by their tendency to volatilize into the air.

Solvents are used for a number of purposes, including manufacturing and cleaning. In the Central Eastside, solvents are present at high concentrations in less than (<) 1% of the primary aquifer. Solvents are present at moderate concentrations in about 3% of the primary aquifer, and are low or not detected in about 96%.

Water delivered to municipal users is commonly disinfected to help control the presence of microbiological organisms; the disinfection process can lead to the formation of trihalomethanes (THMs). When municipal water enters the ground, for example by landscape irrigation, THMs can be introduced, usually at low concentrations, into the aquifer system. In the Central Eastside, THMs are not present at high concentrations in the primary aquifer. THMs are present at moderate concentrations in about 3% of the primary aquifer, and are low (or not detected) in about 97%.

Apart from solvents and THMs, other VOCs were not detected at high or moderate concentrations in the primary aquifer.

Pesticides with human-health benchmarks

Pesticides are used on lawns, in gardens, around buildings, along roads, and in agriculture to help control unwanted vegetation (weeds), insects, fungi, and other pests. In the Central Eastside, herbicides and insecticides were not detected at high or moderate concentrations in the primary aquifer.

In the Central Eastside, the fumigant dibromochloropropane (DBCP) was detected at high concentrations in about 1% of the primary aquifer. DBCP was detected at moderate concentrations in about 8% of the primary aquifer, and was low or not detected in about 91%. Use of DBCP as a soil fumigant was discontinued in California in 1977 (California State Water Resources Control Board, 2002).

BENCHMARKS FOR EVALUATING GROUNDWATER QUALITY

GAMA’s Priority Basin Project (PBP) uses benchmarks established for drinking water to provide context for evaluating the quality of groundwater. Some benchmarks are for protecting human health and others are for protecting aesthetic properties, such as taste and odor. When available, regulatory benchmarks such as Maximum Contaminant Levels are used. Otherwise, non-regulatory guidelines such as Health Advisory Levels are used as benchmarks. Although the PBP uses benchmarks for providing context, groundwater samples were collected prior to treatment or blending, and therefore might not represent the quality of drinking water.

High, moderate, and low concentrations are defined relative to benchmarks

Concentrations are considered high if they are above a benchmark. For inorganic constituents, concentrations are moderate if they are above one-half of a benchmark. For organic and special-interest constituents, concentrations are moderate if they are above one-tenth of a benchmark; a threshold of one-tenth was used because organic constituents are generally less prevalent and have smaller concentrations relative to benchmarks than inorganic constituents. Low values are those that are neither high nor moderate and include non-detections as well as detections. Methods for evaluating proportions are discussed in Landon and others (2010).