In the North-Central subunit, the expense of pumping deep ground water reduces the amount of irrigation contributing to recharge. The Central Columbia Plateau's highest rate of precipitation is more than 20 inches per year in the Palouse subunit. Recharge moves nitrate primarily into shallow ground water. On the other hand, leakage from irrigation canals may lower nitrate concentrations. Nitrate concentrations in recharge are probably controlled by the amount of nitrogen fertilizer applied to the land, the chemical form of nitrogen that is applied, the rate of recharge, and the rate of nitrogen uptake by the crop.
Agricultural fertilizers are the main source of nitrate in ground water.
Man-made fertilizers became widely available after World War II and quickly came into use across the Nation. The extent and intensity of agriculture in the Central Columbia Plateau make high annual rates of fertilizer application a particularly strong influence over water quality. Other sources of nitrogen, such as livestock production, food processing wastes, and sewage, are lesser influences at a regional scale. Nitrogen, ammonia, and ammonium fertilizers break down into nitrate. Nitrate is the form of nitrogen that plants can assimilate.