An investigation of inorganic nitrogen compounds in the groundwater in the Valley of México

Abstract

A study of nitrogen compounds in the groundwater in the Valley of Mexico was prompted primarily by the closing of a series of wells in the southern part of the valley due to high ammonium concentrations. The study attempted to determine if the source of nitrogen compounds in the groundwater was from anthropogenic 50urces, namely wastewater (aguas negras). Wastewater is conducted throughout Mexico City in a network of unIined canals and is also used to irrigate agricultural fields within the valley. Nitrogen compounds were studied in particular because they are a widely recognized constituent of domestic waste and their geochemical processes are reasonably well understood. The principal known risk of nitrogen compounds in drinking water is methaemoglobinaemia, a health hazard to young children. A drinking water limit for nitrate of 10 mg/I as elemental N is recommended by the World Health Organization. Contaminated surface water and well water from various locations in the valley were sampled and analysed for geochemical and environmental isotope parameters. The results were compared in an attempt to identify the presence of the surface water in the well water. Unfortunately the characteristics of the surface water were generally not different enough from the well water to determine if widespread anthropogenic contamination was occurring anthropogenic contamination in recharge areas appears to be indicated by elevated nitrate concentrations in these areas. Anthropogenic contamination in the southern part of the valley (in the vicinity of Ramales Tecomitl. Tlahuac and Santa Catarina) was demonstrated by high ammonium concentrations and the presence of faecal coliforms in the groundwater. Althollgh wells completed belo,,"" the clay in the central part of the valley do not appear to be contaminated, we should not assume that the clay is impermeableand therefore offers complete protection from surface sources of contamination.