Mass Solute Balance and Evaporation Mark Wiltermuth NDSU Geol 628 Geochemistry 2010
Heagle, D., M. Hayashi and G. van der Kamp (2007). Use of solute mass balance to quantify geochemical processes in a prairie recharge wetland. Wetlands 27:
Subject: northern prairie recharge wetland Objective: Identify key geochemical processes and quantify their rates Method: Combined water and solute mass balance approach Key Processes: Sulfate reduction, carbonate mineral reactions, and processes adding CO 2 to the pond
Water chemistry affects the plant and animal communities of a wetland Salinity influences the plant and invertebrate community Source of salinity: Glacial till; oxidation of sulfur and dissolution of carbonate
Three types of wetlands: recharge, flow- through, discharge Hydrologic cycle of closed basins Inflow: Precipitation and Runoff, and Groundwater Outflow: Evaporation, Groundwater Chemical transport: infiltration carries solutes into groundwater Evaporation: deposit solutes, oxidize reduced species
Water balance: Δ Volume = Area ( Precip + Runoff – Evap – Infilt) Use of a conservative species as a tracer (Chloride) Groundwater inflow and outflow Sulfur redox reactions
Normalized Masses of species to first observed concentration to compare to Chloride Changes in Chloride reflect changes do to hydrology Differences between normalized mass of other species indicates reactions Solute mass balance: [ P(C p ) + R(C R ) – f I {I + E}(C) + B ]
Use Chloride to find f I because B=0 for conservative species Can now solve for I and E (so just solved a hydrology problem) Use mass balance for other species, change B to represent the addition or removal of species by reactions
How does evaporation alone change the water chemistry? How can the water chemistry changes be modeled using PHREEQC?
Evaporation by reaction TITLE Seasonal Wetland 25% Evaporation SOLUTION 1 Initial Water 11-May 1994 units mg/L pH 7.18 temp 18.0 Ca 28 Mg 11 Cl 4.5 S(6) 2.56 Alkalinity167 as HCO3 REACTION 1 H2O moles
% Evap Anhydrite CaSO Aragonite CaCO Calcite CaCO CH4(g) CH CO2(g) CO Dolomite CaMg(CO3) Gypsum CaSO4:2H2O H2(g) H H2O(g) H2O -1.7 H2S(g) H2S O2(g) O Sulfur S
% Evap SO CaSO MgSO HSO CaHSO
% Evap HCO CO CaHCO MgHCO CO CaCO MgCO
Modeling precipitation, runoff and evaporation Changes in chemistry through transport in a wetland system