Mass Solute Balanceand Evaporation

Mark WiltermuthNDSU 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: 806-818.

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 CO2 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: ΔVolume = 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(Cp) + R(CR) – fI{I + E}(C) + B ]

Use Chloride to find fI 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 reactionTITLE Seasonal Wetland 25% EvaporationSOLUTION 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 Alkalinity 167 as HCO3

REACTION 1 H2O -1.0 13.875 moles

% Evap 1 5 10 25 50 75 95Anhydrite CaSO4 -3.67 -3.64 -3.6 -3.47 -3.19 -2.74 -1.83Aragonite CaCO3 -0.79 -0.76 -0.71 -0.57 -0.27 0.23 1.26Calcite CaCO3 -0.64 -0.61 -0.57 -0.43 -0.12 0.38 1.41

CH4(g) CH4-

122.81-

122.82-

122.85-

122.93-

123.09-

123.59-

123.98CO2(g) CO2 -1.99 -1.98 -1.95 -1.87 -1.7 -1.39 -0.68Dolomite CaMg(CO3)2 -1.43 -1.37 -1.28 -1 -0.39 0.61 2.68Gypsum CaSO4:2H2O -3.43 -3.4 -3.36 -3.23 -2.95 -2.5 -1.59H2(g) H2 -36.9 -36.91 -36.92 -36.96 -37.04 -37.24 -37.52H2O(g) H2O -1.7 -1.7 -1.7 -1.7 -1.7 -1.7 -1.7

H2S(g) H2S-

124.01-

124.03-

124.06-

124.15-

124.34-

124.89 -125.4O2(g) O2 -11.82 -11.81 -11.78 -11.7 -11.54 -11.13 -10.58Sulfur S -93.08 -93.09 -93.11 -93.16 -93.26 -93.61 -93.85

% Evap 1 5 10 25 50 75 95

SO4-2 -4.62 -4.61 -4.58 -4.51 -4.36 -4.09 -3.49

CaSO4 -5.74 -5.71 -5.67 -5.54 -5.26 -4.81 -3.90

MgSO4 -5.91 -5.88 -5.84 -5.71 -5.43 -4.98 -4.06

HSO4- -9.96 -9.95 -9.93 -9.86 -9.71 -9.46 -8.91

CaHSO4+ -12.16 -12.13 -12.09 -11.95 -11.66 -11.17 -10.14

% Evap 1 5 10 25 50 75 95

HCO3- -2.56 -2.55 -2.52 -2.44 -2.27 -1.98 -1.30

CO2 -3.38 -3.36 -3.33 -3.26 -3.08 -2.78 -2.07

CaHCO3+ -4.79 -4.75 -4.71 -4.56 -4.24 -3.71 -2.56

MgHCO3+ -4.97 -4.94 -4.89 -4.75 -4.43 -3.89 -2.73

CO3-2 -5.70 -5.68 -5.65 -5.57 -5.39 -5.08 -4.37

CaCO3 -5.91 -5.88 -5.84 -5.70 -5.39 -4.90 -3.87

MgCO3 -6.34 -6.31 -6.26 -6.12 -5.82 -5.32 -4.28

Modeling precipitation, runoff and evaporation

Changes in chemistry through transport in a wetland system