Dealing with the Declining Quality of California Groundwater

Posted on: May 13, 2015

If you are involved in California agriculture today, water is a concern you’ve had for several years. The drought and lack of water regulations have resulted in a rapid increase in well drilling, making it difficult for California well drillers to keep up with demand. The result has been a steady decline of standing water levels and declining water quality. While it is not uncommon to see ponding water in our fields after an irrigation or rain event, it is common to be more concerned with field operations and compaction of the soil rather than the lack of deep soil moisture. Without soil moisture in the rooting zone, our crops struggle to meet their evapotranspiration (ET) demand, along with their nutrient uptake needs, because nutrients cannot be taken up when adequate water is not present. Ponding water should be taken as evidence that soil and water quality are making water holding capacity and infiltration rates decline below crop needs, which requires action. Listed below are common interpretive guidelines for determining water quality and appropriate amendments to address these issues.

Water Quality Interpretations

  • As the Sodium Absorption Ratio (SAR) and exchangeable sodium increase, soil particles become more attracted to water and disperse more rapidly, slowing water infiltration.
  • SAR values should not exceed 5 times Electrical Conductivity (E.C.) values.
  • Na should not exceed 3 meq/L, and SAR should not exceed 6.
  • HCO3 may tie up cation nutrients above 1.5 meq/L and has been reported as toxic to plants at much higher levels.
  • pH should be kept between 6.5 and 7.
  • Iron and manganese may induce biological growth above 0.1 ppm in drip systems.
  • Total dissolved solids above 500 ppm may contribute to clogging of drip systems.

Water Amendments Guide

  • Apply an acid forming amendment such as NPhuric or Sulfuric acid when:
    • pH is above 7.5
    • E.C. is below 0.5
    • HCO3 is above 2.5
    • Soil is moderate to high in free lime
    • Ponding or lack of deep infiltration is noted
  • Apply Gypsum to irrigation water when:
    • SAR is greater than 5 times E.C.
    • E.C. is below 0.5 dS/m
    • Sodium exceeds 80 ppm
    • Ponding or lack of deep infiltration is noted

- Ben Cantrell, NorCal Agronomist