Impact of Climate Variability on the Hydrogeochemistry of Ecologically Important Prairie Wetlands and Lakes

Abstract:

The Prairie Pothole region encompasses 715,000 km² of the north central US and south central Canada and contains millions of small wetlands and lakes. It sustains large populations of shore birds and migratory waterfowl. PPR ecology is influenced by wetland geochemistry, which can range dramatically over short distances (≤ 200m) from dilute Ca²⁺-HCO₃^- to saline Na⁺-Mg²⁺-SO₄^2- compositions. These compositional differences result, in part, from long-term critical zone processes in upland areas coupled to groundwater inflow, but they are also influenced by climate. Climate impacts on the geochemistry of 167 wetlands/lakes from a 9700 km² area of North Dakota (USA) were studied. The chemical composition of these wetlands was determined in the early 1970’s during slightly dry climatic conditions and again in 2012-2013, a period of exceptional precipitation. Dilution dominated wetland geochemical trends. Concentrations of Cl^-, Na⁺, K⁺, and Mg²⁺ generally decreased in 2012-2013 compared to earlier data. In contrast Ca²⁺ increased, and SO₄^2- change was variable. The processes driving these modifications were evaluated using inverse (mass balance based) geochemical modeling. The decrease in the largely inert ion, Na⁺ by rainwater addition was used to approximate the net dilution factor of the wetlands which ranged to >9. This volume increase was associated with large expansions of wetland area determined from time-series Landsat data. Introducing dissolution of authigenic CaCO₃, a known constituent of wetland sediments, matched the observed Ca²⁺ increase. Addition of SO₄^2--enriched groundwater (composition determined from well analyses) was required to model wetlands with increased SO₄^2-. Those wetlands with increased SO₄^2- had more negative δ³⁴S_SO4 values, a result consistent with a previously established isotopically light marine pyrite source for groundwater SO₄^2-. Understanding the evolution of wetland chemistry may aid in assessing future climatic impacts to the PPR.