Effects of agrochemicals on surface waters and groundwaters in the Tunga-Kawo (Nigeria) irrigation scheme

Abstract

The Tunga-Kawo Dam has a reservoir capacity of 22 Mm3, and was designed to irrigate 900 ha of land in the Midland region of Nigeria. This paper examines the quality of surface water and groundwater within the scheme. In particular, the seasonal variation of the concentrations of nitrate, phosphate, dissolved oxygen and hydrazine during the year 2000 is discussed. Three sample points: SF1, SF2 and SF3, were selected for monitoring the surface water quality at the upstream, impounded water and downstream sections, respectively. The quality of groundwater was monitored using samples from a well near the dam (GW2) and the downstream section (GW3). Water samples were taken on a weekly basis from the sample points during the year 2000. It was found that the concentrations in the surface water were higher than the WHO standards for drinking water. For example, the nitrate level in SF3 increased from 0.0 mg l-1 before application of fertilizer to 74.1 mg l-1 after fertilizer was applied, while the phosphate level rose from 1.2 mg l-1 to 19.2 mg l-1 during the same period. Similarly, the level of hydrazine increased from 62 μg l-1 to 102 μg l-1. In particular, the concentrations of the determinants in the samples from the downstream section exceeded those in the impounded water and water from the upstream section. In addition, the level of DO at the downstream section SF3 was lower than that of the upstream section. The level was lower than the minimum level required to support a balanced population of desirable flora and fauna. This difference was attributed to the agrochemicals used by the farmers. Although the quantity of fertilizer applied by the farmers is below the quantity required for optimum yields, the excess chemicals in surface and groundwater could be attributed to the techniques and timing of application. The magnitude of these impacts can be reduced by efficient methods of source control of the pollutants.