Variation of groundwater salinity in the partially irrigated Amudarya River delta, Uzbekistan

The Amudarya delta region contains surface and groundwater resources that discharge into the shrinking Large Aral Sea and ultimately control its future fate. These freshwater resources are prerequisites for sustaining the population of the region. However, salinization and pollution caused by agricultural irrigation is a key problem for these water systems. Here, we report results from a recent field measurement campaign conducted during April 2005 which included 24 monitoring wells located in an irrigated region of the Amudarya delta, thereby extending the historical data set of groundwater levels and salinity measurements. This data set is combined with corresponding data from a downstream, non-irrigated region that was formerly irrigated (together covering 16,100km2 between the Uzbek cities of Nukus and Muynak). This comparison shows that in the downstream region, which is currently not irrigated, shallow groundwaters are far more saline (average 23g l− 1) than the currently irrigated region (average 3g l− 1). We estimate that the unconfined aquifer within the 13,500km2 non-irrigated zone of study area contains 9billion tons of salt, or almost as much salt as the entire Aral Sea (containing 11billion tons of salt and covering an area of 20,000km2 in year 2000). Within the non-irrigated zone, there are statistically significant large-scale spatial correlations between groundwater salinity and distance to the Amudarya River, irrigation canals and surface water bodies when distance is measured along the modelled regional groundwater flow direction. Generally, groundwater salinities are lower downstream of surface water bodies in the non-irrigated zone. Annual fluctuations in groundwater salinity are too large to be explained by input from surface water (Amudarya) or wind-blown salt from the dried Aral Sea sediments. Salt transport by groundwater is the only plausible remaining explanation for these changes.