Environmental isotopic evidence for the origins of ground and surface water in a prairie discharge wetland

Abstract

Ground, surface, and soil waters from a 500-ha wetland surrounding Lunby and Stewart Lakes, approximately 24 km northwest of Grand Forks, ND, have a large range of δD and δ18O values and appear to be mixtures of infiltrating modern meteoric water, evaporated modern meteoric water, and ancient water upwelling from the artesian regional aquifer system (“Dakota aquifer”), which subcrops some 30–50 m beneath the low-permeability glaciolacustrine silty clays underlying the wetland. The Dakota component is brackish and has δD≈−135%., δ18O≈−18%. Its isotopic composition indicates that it probably originated primarily as Pleistocene glacial ice. The stable isotope evidence suggests that shallow ground waters are richest in modern meteoric and evaporated meteoric water, but also that a substantial component (up to ≈65%) of Dakota water has reached the vicinity of the water table at some well sites. Upwelling and evaporation of this Dakota water probably account for the high salinity of soils and some shallow ground waters in the wetland. Although δD and δ18O values indicate that the proportion of modern meteoric water decreases with depth, a modern meteoric component has evidently penetrated to at least 13 m, the depth of the deepest borings on site. The deep penetration of modern water in spite of dominantly upward hydraulic gradients is also indicated by the presence of bomb tritium in one of the deeper monitoring wells. The wetland’s budget of surface and soil waters seems currently to be dominated by meteoric components; ground water upwelling from the Dakota aquifer is evidently subsidiary.