Chemical connectivity and multi-element composition of groundwater in depressional wetlands

Little is known about the element composition of groundwater along flow paths between wetlands. What is known is based on a few major elements, such as Na and Ca. We examined the spatial and temporal variation of elements in a depressional-wetland, groundwater-flow system in the Prairie Pothole Region of North Dakota, USA. Wetlands of the region are characterized by their occurrence in hydrologically connected complexes, where those at higher elevations in the groundwater flow system often recharge groundwater (recharge wetlands), while those at lower elevations receive groundwater (discharge wetlands). The aim of our study was to get a better understanding of factors affecting the distribution of more than 30 elements in groundwater along a flow path between recharge and discharge wetlands. We found that oxidation-reduction potential (ORP) decreased as electrical conductivity (EC) increased along the flow path. As EC increased, so did the concentrations of major ions, such as Na, Ca, and Mg. That was already well known. Less known, however, was that concentrations of Cs, Sn, U and Zr increased along the flow path as well. Not reported before was that concentrations of Sn increased strongly as ORP decreased. The concentrations of most elements in groundwater increased with lower relative elevation in the groundwater flow system, but notable exceptions were Ba and Zn, which both showed opposite patterns. Our results contribute to a better understanding of element cycling in groundwater between wetlands, which in turn is important for our understanding of resultant influences on associated biogeochemistry and ecosystem services.