The spatial distribution of nitrate and phosphate in Sempor Reservoir, Kebumen Regency, Central Java

Abstract

Sempor Reservoir is used for irrigation, domestic, industrial, and tourism purposes, as well as hydroelectric power plant and inland fish farming in pens. The land use in its catchment area is mainly plantation (74%) and, followed by, rice field (8.13%), dry farmland (7.74%), and settlement (5.69%). Agricultural activities, both in irrigated and rainfed farming systems, apply fertilizers to maintain crop quality, but when these substances washed away by surface runoff and transported through the drainage system, they end up increasing nutrients in water bodies. Excess nutrients like nitrate and phosphate induce a rapid growth of algae and aquatic plants, disrupting the ecosystems. This study was designed to identify the spatial distribution of nitrate and phosphate in Sempor Reservoir by field observation, water sampling, and laboratory analysis. The water was collected from 25 sampling points distributed at the outlet (close to the spillway), in the middle and the upstream of the reservoir, and around the fish pens (near the check dam). The results showed that nitrate was present on the reservoir water surface and at a depth of 1 meter at varying concentrations, i.e., <0.01-0.36 mg/L. The phosphate levels were <0.01-0.31 mg/L on the surface and <0.01-4.78 mg/L at a depth of 1 meter. The upstream had very high nitrate concentrations both on the surface and at a depth of 1 meter, while the outlet had very high content only at a depth of 1 meter. As for phosphate, very high concentrations were identified in the middle surface of the reservoir (near the pier) and at 1m depth around the fish pens.