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Criteria for Nitrate-Sensitive Areas

Map 2 The first step in implementing these recommendations was to decide which locations in the state should be given highest priority. Two conditions indicate an urgent problem: First, a high level of nitrate contamination in groundwater and, second, a population that depends on that water for drinking.

Those two conditions depend on various factors. University of California soil scientists originally listed five criteria for nitrate-sensitivity of an area:

Groundwater use. Nitrate concentration is critical if groundwater is used for domestic or animal drinking supplies. If it is used only for cleaning, cooling or irrigation of most crops, there is less concern.

Soil type. Sandy or other coarse-textured soils transmit water downward more rapidly, and nitrate with it. Also, these soils are less likely to create conditions in which nitrate turns to a gas and escapes from the soil (denitrification).

Irrigation practices. Inefficient irrigation systems that lead to large volumes of subsurface drainage increase the leaching of nitrates. Typically, these are surface flow systems with long irrigation runs. Well-managed sprinkler or drip systems, or surface flow systems with short runs, reduce the threat of nitrate leaching to groundwater.

Type of crop. Crops most likely to increase nitrate leaching are those that (1) need heavy nitrogen fertilization and frequent irrigation, (2) have high economic value, so the cost of fertilizer is relatively small compared to revenue produced (3) are not harmed by excess nitrogen and (4) tend to take up a smaller fraction of the nitrogen applied. Many vegetable, fruit, nut and nursery crops fit these criteria, and therefore have more potential for nitrate leaching. Those with less potential include field crops such as alfalfa, wheat and sugar beets.

Climate. High total rainfall, concentrated heavy rains and mild temperatures lead to more leaching of nitrates.

Two more criteria for nitrate-sensitivity were developed by FREP:

Distance from the root zone to groundwater. Less distance means a more immediate problem.

Potential impact. This depends on such factors as population density and availability of an alternate water supply.

These seven factors -- groundwater use, soil and crop type, irrigation practices, climate, distance to groundwater, and potential impact--indicate the nitrate-sensitivity of an area. They determined where FREP's initial field activities were directed. In general, two regions of the state, the Central Coast valleys and parts of the east side of the Central Valley, fit the above criteria (see Map 2).

The Central Coast valleys are major vegetable producing areas. In this region irrigated vegetable fields are a potential source of groundwater contamination. The five major crops are lettuce, broccoli, cauliflower, celery and strawberries. These crops account for 41% of the vegetable acreage in California excluding processing tomatoes (see Chart 1).

On the east side of the Central Valley, tree fruits and nuts are major crops, including almonds, walnuts, peaches and nectarines, plums and prunes, and citrus. These crops account for 77% of the total state fruit acreage (see Chart 2).

(The Los Angeles basin and surrounding areas where well measurements also indicate a groundwater nitrate problem are no longer a significant farming region. For that reason, FREP's agriculturally oriented program is not active there.)