Risk Assessment of Gypsum Amendment on Agricultural Fields : Effects of Sulfate on Riverine Biota

Abstract

Gypsum (CaSO4 ∙ 2H2O) amendment is a promising way of decreasing the phosphorus loading of arable lands, and of thus preventing aquatic eutrophication. However, in freshwaters with low sulfate concentrations, gypsum-released sulfate may pose a threat to the biota. To assess such risks, we performed a series of sulfate toxicity tests in the laboratory and conducted field surveys. These field surveys were associated with a large-scale pilot exercise involving spreading gypsum on agricultural fields covering 18% of the Savijoki River catchment area. The gypsum amendment in such fields resulted in about fourfold increase in the mean sulfate concentration for a 2-month period, and a transient, early peak reaching about 220 mg/L. The sulfate concentration gradually decreased almost to the pre-gypsum level after 3 years. Laboratory experiments with Unio crassus mussels and gypsum-spiked river water showed significant effects on the foot movement activity, which was more intense with the highest sulfate concentration (1,100 mg/L) than with the control. The survival of the glochidia after 24 and 48 h exposure was not significantly affected by the sulfate concentrations up to 1,000 mg/L. Neither was the length growth of the moss Fontinalis antipyretica affected. The field studies on benthic algal biomass accrual, mussel and fish density, and Salmo trutta embryo survival did not show gypsum amendment effects. Gypsum treatment did not raise the sulfate concentrations even to a level just close to critical for the biota studied. However, as the effects of sulfate are dependent on both the spatial and temporal contexts, we advocate water quality and biota monitoring with proper temporal and spatial control in rivers within gypsum treatment areas.