Fig. 1. Location of the Norman Landfill, Norman, Oklahoma, showing the vegetated riparian zone, slough, water level monitoring wells and multi-level cluster wells. IC36 and ICSouth are site-specific identification numbers for the Plume and Control wells, respectively. The transect A–A′ is shown in cross-section in Fig. 2.

Fig. 2. Simplified cross-sectional view of the Norman Landfill, showing the leachate plume, uncontaminated groundwater, and approximate positions of sampling sites for this study.

Fig. 3. Isotopic composition of water samples from all depths of the Plume and Control wells from the January 2000 sampling. The local meteoric water line (δ²H = 7.5 δ¹⁸O + 11.7) and slough evaporation trend (a regression based on 29 samples) are shown. The isotopic composition of leachate, the largest rain event prior to January 2000, and volume-weighted average (VWA) precipitation are shown as postulated end member compositions.

Fig. 4. A. Selected depth profiles for the Plume well, showing the seasonal recharge cycle over one year, with associated SO₄²⁻, Cl⁻ and NO₃⁻ concentrations with depth in the well. The arrows show the depth of the isotopically distinct September 1998 storm recharge. B. Depth profiles from the Control well, for the same sampling dates, showing SO₄²⁻ and Cl⁻ concentrations with depth in the well and the depth of the September 1998 storm recharge. Note the difference in scale of the SO₄ axis compared to the Plume well.

Fig. 5. Depth profiles showing SO₄²⁻ concentration and δ³⁴S values of SO₄²⁻ at each sampled depth in the Control and Plume wells. The samples were collected in October of 2004, at the end of the growing season.

Fig. 6. A. Diagram showing relative SO₄²⁻ concentrations with depth over the study period in the Plume well, October 1998–May 2000. Also shown are the height of the water table and the position of the interface between recharge water and underlying leachate, as determined by δ²H offset, Cl⁻ and specific conductance. B. Diagram showing relative SO₄²⁻ concentrations with depth over the study period in the Control well, May 1998–May 2000. Also shown are the height of the water table and the position of the interface between recharge water and underlying groundwater as determined by δ¹⁸O offset and Cl⁻.

Sulfate net input (oxidation and infiltration) and net loss (reduction) rates for the shallow aquifer at the Norman Landfill site

Volume of sediment was assumed to be a cylinder of aquifer with radius 7.7 cm, height based on the number of saturated well screens, and porosity of 0.4. Sampling periods with no data are designated with –. Rates were calculated both as depth-integrated, in millimoles per liter groundwater per day (mM d^− 1) and per volume of sediment, in millimoles per cubic decimeter of saturated sediment per day (mmol dm^− 3 d^− 1).

Sulfate reduction rates from selected previous studies compared with rates from this study

Rates given in publications were in various units; if necessary, these were converted to mass per unit time or mass per volume sediment per unit time for comparison (see footnotes).