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A shallow unconfined glaciofluvial aquifer in North Dakota (USA) has largest groundwater sulfate concentrations near the bottom boundary. A deltaic silt layer underlying the aquifer, at >16 m, is the modern proximate sulfate source for the aquifer. The original sulfate source was pyrite in the organic-rich shale component of the aquifer and silt grain matrix. An oxidizing event occurred during which grain-matrix pyrite sulfur was oxidized to sulfate. Thereafter the silt served as a 'conserving' layer, slowly feeding sulfate into the lower part of the aquifer and the underlying till. A method was developed for estimating the approximate initial sulfate concentration in the source layer and the redistribution time since the oxidizing event, using a semi-generic convection-dispersion model. The convection-dispersion model and a model for the evolution of modern sulfate δS in silt-layer pore water from the initial grain-matrix pyrite δS, both estimated that the oxidizing event occurred several thousand years ago, and was likely related to the dry conditions of the Hypsithermal Interval. The silt layer also serves as an arsenic source. Results indicate that deltaic silts derived from organic-rich shale parent materials in a glacial environment can provide long-term sources for sulfate and arsenic and possibly other related oxidative weathering products. [ABSTRACT FROM AUTHOR]

Rising water tables, increased soil salinity, and poor trafficability have prompted rapid expansion of subsurface drainage in the Red River Valley of the North in eastern North Dakota and northwestern Minnesota. A conventional subsurface drainage (CD) and subirrigation (SI) field study was conducted in southeast North Dakota from 2008 to 2010 to investigate drainage and subirrigation effects on water quality. Water samples were collected biweekly from a sump pump structure (used as the water inlet and outlet) and 16 observation wells within the field. Water quality variables included chloride (Ct), electrical conductivity (EC), total dissolved solids (TDS), sodium adsorption ratio (SAR), sodium (Na+), orthophosphate (PO4-P), ammonium (NH4-N), nitrite and nitrate (NOX-N), Kjeldahl nitrogen (TKN), and total nitrogen (TN). A three-factor partially nested design and a general linear model with random effects were employed to compare the effects of water management treatment, distance to drain, and well locations (soil heterogeneity) on water quality. The most significant water quality difference was found at the outlet structure, where a significant difference (p < 0.001) between the CD and SI water was found for all ten variables. The water quality of the drainage water was better than the subirrigation water from the aquifer, except for the NOX-N, EC, and TDS concentrations. Well water Ct concentrations inside the field were significantly greater in SI compared with CD water; EC, TDS, SAR, and Na+ were not. In contrast, EC, TDS, SAR, and + were significantly higher at two well locations, indicating that soil heterogeneity affected the water quality. Due to SI practice, a significant difference for Ct, SAR, and Na+ was found between the locations closest to and farthest from the drains during the SI practice, which implies that the SI process may cause soil properties to change in the future. Overall, well locations significantly affected PO4-P, NOX-N, and TN, indicating that the soil physical and chemical properties affected the water quality, and these effects could overcome the difference due to different water treatments. [ABSTRACT FROM AUTHOR]

A field experiment was conducted during the 2009 and 2010 growing seasons to determine the effect of subsurface drainage (SSD) on evapotranspiration (ET) and crop coefficients () for a farm field in the Red River Valley of North Dakota. The total area of the field was 44 ha, half of which had subsurface drainage installed in the fall of 2002 at an approximate depth of 1.1 m and a spacing of 18.3 m. Corn ( Zea mays) was planted in 2009 and soybean ( Glycine max) in 2010. Evapotranspiration rates were measured in both the SSD and surface drained [or undrained (UD)] by using the eddy covariance (EC) method. The changes in water table and soil moisture content were monitored continuously in both fields. The for corn and soybean was developed by using the ET measured by the EC system, and the reference ET was estimated by using the American Society of Civil Engineers Environmental and Water Resources Institute alfalfa reference method. As expected, the use of SSD affected the ET in a seasonal pattern and the ET was crop dependent. Seasonally, higher ET was observed during spring and fall in the UD field attributable to shallower water table and higher soil moisture content. In the summer, a higher ET was found in the SSD field. The higher ET in the UD field in spring and fall, which was 109 and 191 mm in 2009 and 2010, compared with 105 and 176 mm in 2009 and 2010 in the SSD field, did not offset the higher ET in the SSD field in the summer, which was 310 and 351 mm in 2009 and 2010, compared with 249 and 324 mm in 2009 and 2010 in the UD field. For July and August, the ET in the SSD field was 31% greater in 2009 for corn and 14% greater in 2010 for soybean than that in the UD crop fields. For the entire growing season, the ET in the SSD field was 16% higher in 2009 and 7% higher in 2010 compared with the UD field. During the peak growing season (July), the was greater in the SSD field, with peak values of 0.70 for corn and 0.76 for soybean, but in the UD field, the peak values were only 0.54 for corn and 0.65 for soybean. [ABSTRACT FROM AUTHOR]