Groundwater flow system and water quality in a coastal plain aquifer in northwestern Nicaragua

The Departments of Leon and Chinandega in northwestern Nicaragua are agricultural regions that were under cotton cultivation for almost thirty years (1950's--1980's), and more recently mainly under sugarcane. Water supply for the population of almost 700,000 inhabitants comes mainly from groundwater. The principal aquifer in the region is a shallow unconfined alluvial unit underlain by fractured volcanic rocks. Several hydrogeological and chemical studies have identified significant water quality impacts by pesticides and nitrate, but none of the studies have investigated the depth of impact in the aquifer. This information is important in order to ensure new potable water supply wells are drilled deep enough and to ensure the unimpacted deeper aquifer is protected in the long-term. Monitoring wells were installed at five different depths at three locations roughly along the groundwater flow direction. Precipitation, groundwater, and surface water samples were collected during two sampling events. Agrochemical impact in the aquifer was observed at depths of up to 12 m below the water table at all three sites and originates mostly from the historical application of pesticides in the cultivation of cotton. Isotope composition confirmed that in general groundwater at increasing depth in the aquifer is recharged at increasingly higher elevations although some mixing of local and regional flow systems is evident. This has important implications in terms of aquifer protection and management strategies.