The short answer is not very much (unless you are fertigating). Nitrate-N concentrations in most agricultural wells on the Central Coast range from 2 to 50 ppm. Some wells do have background levels of NO3-N as high as 70 ppm but these concentrations are uncommon.
We used an electrical conductivity (EC) meter to measure the change in salinity of well water after adding different concentrations of nitrate salts. The well water tested in our study was highly suitable for vegetable production: the EC was 0.5 dS/m and the background concentration of nitrate was 3 ppm NO3-N. We added sodium- and calcium-nitrate salts to the water to achieve concentrations of nitrate ranging from 3 to 42 ppm N.
Figure 1 shows the effect of nitrate concentration on salinity of the water. The salt content of the water did increase with increasing concentrations of NO3-N, but increases in salinity were modest. For each increase of 10 ppm NO3-N, the EC of the water was raised by 0.07 dS/m. At the highest NO3-N concentration evaluated (42 ppm N) the corresponding EC of the water averaged 0.77 dS/m, or a boost in salinity of 0.27 dS/m. Even at the highest nitrate concentration, the water that we tested was still very suitable for producing cool season vegetables and strawberries without causing yield loss.
Figure 1. Nitrate concentration effects on salinity of water.
A high concentration of nitrates in irrigation water can also supply a significant portion of the nitrogen needs of a crop. Irrigation water with an NO3-N concentration of 42 ppm would supply approximately 9.5 lbs of N/acre for each inch of water applied, or about 77 lbs of N/acre for a lettuce crop if 8 inches of water were applied during the season.
Some growers have observed that their high nitrate wells are problematic for producing good vegetable crops, and have remarked that high nitrate water tends to be salty. This study demonstrated that nitrate is probably not the cause of the salinity, unless the nitrate concentrations are very high (> 100 ppm N).
Well water can have a high salt content for a variety of reasons. Underground aquifers derived from marine sediments can contain salts that increase the salinity of groundwater. Near the coast, wells can become salty if seawater intrudes into ground water. Another source of salts is from irrigation water. Approximately 1 ton of salts are contained in an acre-foot of water that has an EC of 1.15 dS/m. As crops transpire moisture, salts accumulate in the soil, and will eventually leach deeper in the soil profile as more irrigation water is applied. Over time, leached salts could eventually reach the ground water, and increase the salinity level of the water.
In conclusion, nitrates in irrigation water are unlikely to be a significant cause of salinity of ground water. If your well water has a high concentration of nitrates and also has a high EC (> 1 dS/m), a water suitability test may reveal other salts contributing to the salinity of the water. Nitrates in the water can also be a significant source of nitrogen for your crops.