Simple solutions for steady–state diffuse recharge evaluation in sloping homogeneous unconfined aquifers by means of atmospheric tracers

Abstract

Environmental tracers of atmospheric origin are widely used to localise geographically recharge to aquifers and to quantify its average value. The most commonly used atmospheric tracers are chloride, water stable isotopes, tritium and radiocarbon. Their average input is spatially variable. Spring and pumped groundwater samples are often mixtures of water from a range of depths and thus their concentration in water may conspicuously differ from the tracer content in local recharge and even the recharge produced at the centroid of some up flow area. In the case of radiocarbon, most interpretation efforts are devoted to know the part of dissolved inorganic carbon originated from biogenic soil CO2 but seldom is considered that the water sample is often a mixture corresponding to a wide range of flow times along different paths in the unsaturated and saturated zones. Environmental tracer content in groundwater samples from large springs and long–screened pumped wells is interpreted and quantified. This is done under simplified idealised conditions, in homogeneous formations and under steady–state flow and mass transport through the unsaturated and saturated zones. Different input forms of the environmental tracer flow and mixing are considered, according with their specific characteristics, assuming linear variations with distance over the territory and three basic geometrical settings: parallel flow, radial divergent flow and radial convergent flow, besides a two-level geometry with homogeneous inputs. Tracer content in samples similar to those from a well-mixed reservoir differs from that of local recharge and depends on geographical position. The quantitative understanding of the slope effect, even if deduced from synthetic cases, improves recharge evaluation by means of chloride, shows how consider water isotopic distribution and altitudinal gradient in the aquifer, and demonstrates that radiocarbon ages may depend on position for a given geometrical shape of the aquifer, besides the large influence of the unsaturated zone thickness.