Quantifying groundwater discharge from different sources into a Mediterranean wetland by using 222Rn and Ra isotopes

Abstract

► 222 Rn and Ra proved to be a suitable tracer of groundwater discharge to coastal wetlands. ► 222 Rn, 226 Ra and salinity allowed estimating the contribution of different groundwater sources. ► Ra isotopes allowed estimating the wetland water age. ► Monthly groundwater inflows from two aquifers were successfully monitored. Groundwater discharge constitutes the main water inflow of many coastal wetlands. Despite the potential of Ra isotopes and 222 Rn as tracers of groundwater discharge, the use of these radionuclides to quantify the groundwater inflow in coastal wetlands has been only scarcely addressed in the literature. The main goal of this study is to evaluate the use of 222 Rn and Ra isotopes to estimate the contribution of distinct groundwater sources into a Mediterranean coastal wetland (the Peníscola marsh, Castelló, Spain). The Peníscola marsh is a small shallow wetland nourished by groundwater coming from four different flowpaths: (i) a deep flow from the regional carbonate aquifer of El Maestrat, (ii) a shallow flow and (iii) an intermediate flow, both from the Irta Range and the detritic Vinaròs-Peníscola aquifer, and (iv) seawater intrusion. Data on 226 Ra, 222 Rn and salinity obtained in summer 2007 revealed that the deep groundwater contribution was 15% of the total water inflow, whereas the shallow and intermediate flow paths represented 32% and 48%, respectively. Seawater accounted only for the remaining 5% inputs to the wetland. Ra isotopes also allowed estimating the marsh water age in 1.2 days. Both the groundwater contributions derived from 222 Rn measurements and the Ra-derived marsh water age agreed well with the direct measurements obtained using propeller flow meters, evidencing the effectiveness of the used methods. An interannual comparison between the estimated groundwater inflow and the precipitation revealed that shallow groundwater flows respond to local precipitation, whereas the deep groundwater flow from the carbonate aquifer is dominated by a constant baseflow.