H52A-01
On the Evaluation of Mixing Ratios and Their Use to Quantify Reaction Rates
Mixing ratios quantify the proportions in which two or more end-member waters are mixed in a sample. The ultimate objective of tracers is to quantify mixing ratios. Their evaluation requires that end-member concentrations are perfectly known. This is rarely the case when dealing with natural tracers. However, much redundant information is often available. Here, we revise a maximum likelihood methodology to estimate mixing ratios, while acknowledging uncertainty in end-member concentrations. The method allows to not only evaluate mixing ratios, but also to obtain improved estimates of end-member concentrations. Mixing drives many of the reactions that take place underground. Therefore, one should expect mixing ratios to not only allow identifying the origin of water, but also to quantify reaction rates. We present a methodology to do just so for the case of equilibrium reactions. The procedure is relatively simple. Together with traditional mixing calculations, provides a natural framework to improve understanding of both the origin of groundwater samples and the chemical processes it has undergone.
H52A-02
Variable Transport of Fluorescent Tracers to Springs of Mantled Karst in the Great Valley of Pennsylvania
Karst springs of south-central Pennsylvania support productive wild fisheries, trout hatcheries, and water supplies, and typically exhibit characteristics of diffuse flow systems. Dye traces are not documented for the region, resulting in uncertainty in both groundwater flow and in potential for runoff contamination. Surface drainages in this part of the Great Valley include the Yellow Breeches and Conodoguinet creeks, each flowing east and north to the Susquehanna. Big Spring, the focus of this study, flows at an average of 868 l/s and northward across the Great Valley to the Conodoguinet. Temperature is relatively constant at 10-11 degrees C, with turbidity typically $<$10 NTU. We released fluorescein (FL) into a losing reach of the Yellow Breeches, 5.2 km to the south, and sulphorhodamine-B (SRB) to a sinkhole in a failing detention basin 8.9 km to the west of Big Spring. SRB traveled to Big Spring within 3.5 days, parallel with geologic strike. West and East source springs of Big Spring responded differently, with a clear SRB peak in the west spring (316 ppt), and less distinct SRB peak occurring in the smaller east spring. Fl did not cross the valley from the Yellow Breeches watershed, but rather was weakly detected one month later 9.5 km to the east at springs of Huntsdale state fish hatchery. Neither dye was detected in springs bracketing Big Spring to the west and east, including a second contributing east spring that serves as water supply. Major springs are fed by separate, regional flow systems along strike, and may receive rapid, regional transport of surface runoff from sinkholes where the colluvial mantle thins. Background fluorescence of spring and well waters is also being analyzed, with particular focus on organic acids from forested source waters and human or animal waste from valley sources.
H52A-03 INVITED
Radium isotope quartet in groundwater as a proxy for identification of aquifer rocks and mechanisms of water-rock interactions: examples from the Negev, Israel
Many aquifer systems are composed of multiple rock types. Previous attempts to evaluate the specific aquifer rocks that control the groundwater chemistry and possible flow paths within these multiple lithological systems have used major ion chemistry and isotopic tracers (e.g., strontium isotopes). Here we propose an additional isotopic proxy that is based on the distribution of radium isotopes in groundwater. Radium has four radioactive isotopes that are part of the decay chains of uranium-238, thorium-232, and uranium-235. The abundance of radium isotope quartet (226Ra-half life 1600 y; 228Ra-5.6 y; 224Ra-3.6 d; 223Ra-11.4 d) in groundwater reflects the Th/U ratios in the rocks. Investigation of groundwater from the Negev, Israel, enabled us to discriminate between groundwaters flowing in the Lower Cretaceous Nubian Sandstone and the Upper Cretaceous Judea Group carbonate aquifers. Groundwater flowing in the sandstone aquifer has distinguishably high 228Ra/226Ra and 224Ra/223Ra ratios due to the high Th/U ratio in sandstone. In contrast, the predominance of uranium in carbonate rocks results in low 228Ra/226Ra and 224Ra/223Ra ratios in the associated groundwater. We show that the radium activity in groundwater in the two-aquifer systems is correlated with temperature, dissolved oxygen, and salinity. The increase of radium activity is also associated with changes in the isotopic ratios; 228Ra/226Ra ratios increase and decrease in the sandstone and carbonate aquifers, respectively. Given that the dissolution of radium isotopes depends on their decay constants, the use of the four radium isotopes with different decay constants enabled us to distinguish between dissolution (higher abundance of the long-lived isotopes) and recoil (predominance of the short-lived isotopes) processes. In spite of these isotopic fractionations, the radium isotopic discrimination between carbonate and sandstone aquifers is significant.
H52A-04 INVITED
Stable isotopes of water and sulfate reveal links between recharge processes and sulfate- reducing zones at a landfill leachate contaminated site, Norman, Oklahoma
Investigations of the interaction between recharge and natural attenuation processes have been conducted at the USGS Norman Landfill research site in Oklahoma. A 12 m thick unconfined alluvial aquifer is contaminated with leachate from a closed, unlined municipal landfill. A two-year field study (May 1998 to May 2000) yielded monthly samples of the saturated zone for water stable isotopes and anions at 15-cm depth intervals in multi-level wells in the top 2 m of the aquifer. Precipitation samples for stable isotope analysis were collected biweekly. Further work involved sampling for $\delta$$^{34}$S analysis of dissolved sulfate in May and October 2004. Isotopic enrichments ($\delta$$^{2}$H in the leachate-impacted groundwater and $\delta$$^{18}$O in evaporated surface water) along with chemical contrasts between groundwater and recharge allowed quantification of infiltrated recharge. Recharge was 37% of rainfall over the study, but the isotopes showed that most recharge water entering the groundwater system in the winter and spring seasons was subsequently removed by phreatophyte transpiration during the growing season, rather than mixing into the deeper aquifer. Dissolved sulfate levels (up to 15 mM) were highest in the fall, following initial recharge events after the growing-season water table decline. A large, isotopically depleted fall rain event allowed tracing of the resulting recharge and its associated high sulfate concentrations as the water moved downward in the aquifer. The observations suggest that sulfides are oxidized by dissolved oxygen in the capillary zone or unsaturated zone during the summer growing season, when the water table drops by 1 m or more, then the resulting sulfate is entrained by the rising water table and pushed deeper in the aquifer by recharge during fall and winter. Microbial sulfate reduction redeposits sulfides during this time. This interpretation was supported by October 2004 sulfate $\delta$$^{34}$S values of $>$30\permil in deeper levels of both contaminated and uncontaminated wells, indicating sulfate reduction had occurred, while a $\delta$$^{34}$S value of -7\permil was observed near the water table in the uncontaminated well, suggesting sulfide oxidation as the source of sulfate. The isotopes were important tools in elucidating complex interactions between recharge, groundwater fluctuations and sulfate reduction that may be coupled to natural attenuation processes in the contaminated aquifer.
H52A-05 INVITED
Contaminated Groundwater N flux to Surface Waters from Biosolid Waste Application Fields at a Waste Water Treatment Facility
Biosolids have been land applied at the Neuse River Waste Water Treatment Plant (NRWWTP) since 1980. The long biosolid application history at this site has resulted in a build up of nitrate in the ground water beneath the Waste Application Fields (WAFs). We have used an innovative river monitoring system that measures in situ nitrate concentrations and discharge above and below the plant to determine the amount of nitrate gained in the reach from the WAFs. The nitrogen and oxygen isotopic composition of nitrate in the WAF groundwater indicates that 18% of the monitoring wells are impacted by fertilizer N, 57% of the wells are impacted by biosolid N, 22% of the wells are affected by denitrification, and one well is impacted by A.D.N. The net daily contribution of surface / ground water and nitrate to the reach was calculated from the sum of the flux into the reach at the upper RiverNet station plus the plant discharge minus the flux out of the reach at the lower RiverNet station. The difference between the flux into the reach and plant discharge minus the flux out of the reach is termed the non-point source gain (NPS gain). The NPS gain could come from groundwater and/or surface drainage additions to the reach. On an annual basis, daily integrated NPS nitrate gains were ~70,000 kg in year 2004 and ~27,900 kg in 2005. This represents an average over the two year period of ~12% of the total nitrate flux out of the reach and 43% of the nitrate discharged from the plant. During the past year groundwater wells were installed in the river riparian buffer and N Flux was measured in a surface water drainage in the WAF. The results indicate that N is not migrating through the shallow groundwater, and most of the NPS gains in the reach can come from surface drainages which have nitrate concentrations of 30-80 mg/l. Over the next year wetlands will be reconstructed in the surface drainages to attenuate the N flux and protect river water quality.
<a href='http://rivernet.ncsu.edu'>http://rivernet.ncsu.edu</a>