Date of Award


Degree Name

MS in Agriculture - Soil Science


Earth and Soil Sciences


Terry Smith, Christopher Appel, Trygve Lundquist


Agricultural production in Monterey County, California is a multi-billion dollar industry. Near the coast, seawater intrusion has threatened to degrade the groundwater quality due to over-pumping of the aquifer. The Monterey Regional Water Pollution Control Agency (MRWPCA), in partnership with the Monterey County Water Resources Agency, has provided recycled water since 1998 to over 12,000 acres of prime agricultural farmland in the northern Salinas Valley in an effort to reduce groundwater removal. The dominant soil types in the region are clay loam and clay soils, which are both susceptible to sodium (Na) accumulation and water infiltration problems. Recycled water blended with well water is used to irrigate cool season vegetables (i.e., artichokes, broccoli, Brussels sprouts, celery, cauliflower, and lettuce) and strawberries. A long-term study was implemented by MRWPCA to monitor salinity levels in commercial vegetable fields because of grower concerns that salts in the recycled water would have long term effects on soil quality. Accumulation of salts over time would make the soil less productive. Soil salinity levels were monitored at three Control and three Test Sites beginning in the spring of 2000. The Control Sites received well water, and the adjacent Test Sites received an approximate 2:1 blend of recycled and well water, respectively. Control and Test Sites were paired based on location to compare the same soil, crop, drainage systems, and farming practices. The soil was sampled three times per year from all sites: spring (before planting), mid-summer after harvest of the first crop, and late fall after the second crop harvest. Composites of four cores were collected at each site from the zero to 36-inch depth at 12-inch intervals. Each 12-inch interval soil sample was analyzed for pH, electrical conductivity (ECe), extractable cations (Na+, Ca2+, Mg2+, and K+) and extractable anions (Cl-, NO3-, and SO4-). After 10 years of monitoring, the data showed that using recycled water blended with well water at the Test Sites increased the ECe of the soil profile from 2.1 to 2.5 dS/m and increased the sodium adsorption ratio (SAR) from 3.0 to 3.9. The data also showed that using well water at the Control Sites increased the ECe of the soil profile from 1.4 to 2.6 dS/m and the change in SAR was negligible. The Test and Control Sites were significantly different for ECe and SAR, which was expected considering a higher salt content in the recycled water compared to the well water. The significant differences for ECe and SAR were associated with the significant differences in soil Na+ levels between the Test and Control Sites. The SAR and ECe of soil samples from all sites were in a range acceptable for vegetable production. The use of recycled water for irrigation of cool season vegetables and strawberries in the study area has not shown an indication of degraded soil productivity. Based on vegetable production and the slow increase of salts in the soil, recycled water can be used for long-term irrigation with proper management.