| Geochemical data of waters from the water table (WT) beneath two fields at the Northern Cornbelt Sand-Plain MSEA (Management Systems Evaluation Area) near Princeton, Minnesota were used to test the hypothesis that chemical and physical heterogeneities in the soil and vadose zones produce significant time-spatial variations in the geochemistry of recharge to a shallow WT even in a relatively homogeneous sand plain. Multivariate statistical techniques (factor and cluster analyses) were combined with heterogeneous (e.g., multiphase) equilibria modeling to test the hypothesis. Fifteen geochemical variables (SC, DO, Ca2+, Mg2+, K+, HCO 3−, NO3−, SiO 2, pH, Na+, Mntot, Fetot, Cl −, SO42−, and CO3 2) were studied.; Two variables falsified the working hypothesis. The ion ratio of Ca 2+ to Mg2+ was constant at 2.3 ± 0.25, consistent with a cation exchange buffering of the ratio. The heterogeneous equilibria of recharge waters appeared to be more influenced by the spatially distributed chemical properties of the subsurface than by the time-related variables. SiO2 was constant at 16.4 ± 2.2 mg/l across all of the samples at a level consistent with a chalcedony solubility control. The rest of the variables displayed time and spatial heterogeneity.; The cluster and factor analyses identified three chemical facies: (Ca,Mg)(HCO 3)2, (Ca,Mg)(NO3)2, and (Ca,Mg)(NO 3,HCO3,Cl)2. Five significant factors explain over 80 percent of the sample variance (SV): (1) proton/cation exchange and mineral weathering by carbonic acid (33% of the SV); (2) carbonic acid speciation and competitive exchange involving K+ and H + (15% of the SV); (3) redox processes between Mntot, Fetot and DO (15% of the SV); (4) a NaSO4 component to the recharge (10% of the SV); and (5) independent SiO2 (8% of the SV). Nitrate and chloride from agricultural practices were accompanied by Ca2+ and Mg2+, while SO4 2− was affected by adsorption and/or anion exchange reactions. Concentrations of Na+ appeared to be partly derived from the weathering of plagioclase. Dissolution of carbonates in the WT under a closed system with respect to pCO2 contributed additional solutes to the recharge waters. |
