Sequential variation of groundwater quality in an agricultural area with greenhouses near the coast

The quality of coastal groundwater utilized for greenhouse-farming is liable to be affected by not only fertilization practices but also seawater intrusion into the aquifer. In this study, the sequential variations of groundwater quality and the mechanisms of its pollution processes were analyzed through successive field measurements from June 2000 to May 2001 in such a coastal agricultural area located in Kochi prefecture, Japan. The analysis revealed that the ions for NO₃⁻, SO₄²⁻, Mg²⁺ and Ca²⁺ in the groundwater originated primarily from the ground surface due to the seasonal flooding activities. It was also estimated that denitrification occurred at three well points, and the rate constants were 0.010, 0.019 and 0.089 d^-1, respectively. Na⁺, Mg²⁺ and Ca²⁺ concentrations in the groundwater governed by seawater intrusion phenomena showed strong correlations with the concentration of Cl⁻ by excluding the influence of fertilizer. Cation exchange phenomena between Ca²⁺ and Na⁺ and as well as the mixing with seawater had a considerable effect on the groundwater quality.