Isotope Tracers of Biogeochemical and Hydrologic Processes I
Presiding: T Bullen, U.S. Geological Survey; J Hogan, University of Arizona
H43D-01 13:30h
Temporal Variations in 234U/238U Activity Ratios in Four Mississippi River Tributaries
In 2004 we sampled the four tributaries that are the major contributors to the Mississippi River in terms of water discharge, i.e., the Arkansas, Missouri, Upper Mississippi, and Ohio rivers. Each river was sampled four times over the course of the year at variable levels of discharge in an attempt to constrain the causes of the temporal variations of 234U/238U activity ratios in the lower Mississippi River at New Orleans. The tributary uranium data support the idea that lower river uranium isotope and elemental systematics are controlled by a simple mass balance of the source tributary discharges. Furthermore, the uranium isotope ratios of the individual tributaries show coherent patterns of variability. Specifically, the data obtained from the four sampling trips yielded similar patterns of temporal variation in the 234U/238U activity ratios of all of the rivers, although the absolute values of these ratios were distinctly different from one river to the next. The pattern was such that the highest 234U/238U activity ratios were observed during the highest flow associated with the spring freshet while the lowest ratios occurred during the summer. For example, in the Missouri River, the 234U/238U activity ratios varied from 1.51 (February 12) to 1.37 (April 14) to 1.34 (July 16) to 1.37 (November 12), while in the Ohio River the same ratios varied from 1.36 (February 12) to 1.29 (April 14) to 1.21 (July 16) to 1.23 (November 12). The apparent seasonal pattern of these ratios in each tributary has led to several ideas as to the causes of the observed trends. The first, and most obvious, is that in each individual drainage basin there are various source tributaries that contribute to the uranium isotope systematics of the main stem of the tributary of interest. It follows that the variations in the uranium activity ratios may be caused by spatial variations in the source rock chemistry of the drainage basin. Other more complex scenarios can also be envisioned and will be discussed. For example, we explore the possibility that the highest ratios associated with the spring freshet are a consequence of snow melt and the flushing of 234U from fresh surfaces created via physical weathering associated with the winter freeze-thaw cycles.
H43D-02 13:45h
Using Geochemical Tracers to Quantify Baseflow Inputs to the San Pedro River, Southeast Arizona
Groundwater managers in the semiarid Southwest U.S. must balance the demands of growing populations against the needs of rare riparian ecosystems. The Upper San Pedro River Basin in Southeastern Arizona provides water for the rapidly growing area of Fort Huachuca, Sierra Vista, and surrounding communities. This basin also supports the riparian area by providing baseflow to the San Pedro River and shallow groundwater to phreatic riparian vegetation. Before water managers can properly plan for sustenance of the riparian area, they require answers about the inputs of baseflow into the river. What is the temporal and spatial variability of recharge and discharge? What are the relative inputs of different sources (i.e. recharge of monsoon runoff and inflow of basin groundwater) into the riparian aquifer? We have used a suite of geochemical tracers to address these questions. Most precipitation in this region falls during the summer monsoons and winter frontal storms. Because these events have different source areas, their respective stable isotopic signatures differ. We use these isotopic compositions as end members in mixing equations in order to determine the dominant season for recharge. Groundwater in the basin has a narrow range of stable isotope compositions, varying from 62 to 72% winter precipitation. The basin isotopic composition is very similar to water in the mountain front and block, indicating that recharge from these areas is the dominant source for basin groundwater. Basin groundwater residence times, determined using the radioactive isotope carbon-14, are on the order of 10,000 years or more, indicating a low recharge rate. In contrast, riparian groundwater has a wide variation of isotopic values, indicating that this water is a mixture of basin groundwater and monsoon runoff, varying from 20 to 90% basin groundwater. The dominance of basin groundwater or monsoon runoff in different areas of the river correlates well with independent classification of river reaches as either gaining or losing, respectively. Tritium age data indicate that some riparian groundwater, especially in losing reaches, has been recharged recently (less than 40 years). Baseflow varies from 40 to 100% monsoon precipitation, and is closer to basin groundwater in gaining reaches. These results help can water managers better understand the water resources of the basin, and increase their ability to effectively manage the riparian area. For example, the demonstrated importance of monsoon recharge to the riparian aquifer indicates that the construction of retention basins to capture monsoon runoff could be one effective strategy to increase recharge into the basin.
H43D-03 14:00h
Tracing Cadmium in the Environment: an Evolving Stable Isotope Approach
Cadmium (Cd) is a trace constituent in rocks and waters, and like many transition metals is an essential dietary nutrient at low levels but highly toxic in elevated doses. In many respects, cadmium behaves chemically like calcium (Ca) and thus substitutes for Ca in liquid-solid partitioning reactions and generally follows Ca through biogeochemical cycles and metabolic processes. Cd is comprised of 8 stable isotopes, and given the isotopic systematics of environmental Ca it is likely that variations in the stable isotope composition of Cd in natural materials will result from both inorganic and biologic processes. In order to assess the potential of Cd isotope variations to reveal information about sources, metabolic and biogeochemical pathways and fates of environmental Cd, we have initiated a broad study of the stable isotope composition of Cd in a variety of natural and anthropogenically-influenced systems. As an example, here we report the results of the first systematic study of the stable isotope composition of Cd in biologic materials. We focused on the isotopic variability of Cd in tissues of two species of clam collected from the San Francisco Bay estuary, Potamocorbula amurensis which resides in brackish water and Corbicula fluminea which resides in fresh and slightly brackish water. Both clam species concentrate Cd in their soft and hard tissues. During both low-flow conditions in August and high-flow conditions in April, Δ116Cd of soft tissues of Potamocorbula was consistently negative and increased down-estuary with increasing salinity (Δ116Cd is defined as the per mil difference of the 116Cd/110Cd ratio between a sample and our standard, igneous rock BIR-1). Samples collected in August were systematically displaced to higher Δ116Cd than those collected in April. Soft tissues of Corbicula collected in both August and April from upstream, fully fresh-water sampling sites had identical Δ116Cd, while soft tissues of Corbicula collected from our site at the fresh water-brackish water interface had variable Δ116Cd that was approximately 0.4‰ less negative than that of co-sampled Potamocorbula. These results point to a variety of potential controls on the isotopic distribution of Cd, including differences in aqueous speciation of Cd in saline and fresh waters, species-specific and temporal variations of nutritional sources and distribution of Cd between soft and hard tissues, and changing influences of Cd contamination to the ecosystem.
H43D-04 14:15h
Biogeochemical Processing of N in a Mantled Karst Watershed
Nitrate (NO3-) contamination of aquatic ecosystems is an increasing threat particularly in karst-dominated landscapes where agricultural practices are on the rise. The importance of sub-surface NO3- processing in a karst experimental watershed was assessed by measuring dissolved organic carbon (DOC) bioavailability, geochemical constituents, nitrate concentration and stable isotopic composition. We used a synoptic sampling method aimed at characterizing nitrate processing under base and storm flow conditions along a hydrogeologic gradient. Our results, consistent with previous work, demonstrate the importance of the soil-water interface zone in nutrient processing. However, little is known about the interflow zone above the epikarst surface. Use of a binary mixing model, with chloride as a conservative tracer, suggests a >20% change in NO3- and DIC concentration was due to processing and 69% to dilution during storm flow. Ground water community respiration rate (GWCR)(Μ MC h-1) normalized to DOC (mMC) indicate more bioavailable C for NO3- processing in the interflow zone. Stable nitrogen and oxygen isotopic composition of NO3- should support the interflow zone as an important site for NO3- transformation in this karst watershed. Results demonstrate hydrologic flow conditions alter chemical and organic constituents and therefore biological processing in this karst system.
H43D-05 14:30h
Rapid and Drastic Changes in the 15-N Signature of Forest Soil Nitrate
Recent work has utilized the dual isotopes of nitrate as a tracer in forested catchments. This method provides the relative contribution of atmospheric and microbial sources of nitrate in stream export. Tracing nitrate movement through the soil profile in these ecosystems may be more problematic. Sampling soils to extract nitrate stimulates both net and gross nitrification with the potential to create rapid changes in the 15-N signature. We have found changes in 15-N as high as 16 per mil in soils incubated 2-3 days at cold temperature. The initial 15-N and the change in 15-N both were significantly related to the measured net nitrification rates. In organic horizons with high net nitrification rates, the initial 15-N of nitrate was depleted but increased after sampling toward the signature of the whole soil N. The opposite was true in soils with relatively low net rates; the initial 15-N of nitrate was close to that of the whole soil N and incubation caused depletion. We hypothesize that mixing the soil causes more complete utilization of available ammonium in the former case and provides a greater pool of ammonium in the latter case. The potential for these rapid changes must be recognized when sampling soil for the dual isotope method.
H43D-06 14:45h
Identifying sources and sinks: An investigation of Irish forest soil fluxes of methyl chloride using isotopic analysis
Methyl chloride provides 20% of the inorganic chlorine in the stratosphere and is therefore responsible for approximately 10-15% of the annual ozone loss. Methyl chloride is primarily naturally produced, with minor human induced changes in its global budget. Several key areas within the budget remain relatively unexplored. Atmospheric fluxes of methyl chloride to and from soil ecosystems are poorly understood due to the complexity of the microbial, fungal and abiotic interactions. Isotopic studies have shown their utility in separating source and sink terms for the same compound. Here we present results from analyses of Irish forest soils as an indication of the functionality of this method. Flow-through chambers were used to separate microbial, fungal and abiotic signatures. Surprisingly, we find that the isotopic signature of abiotic production at ambient temperatures is nearly identical to the ambient atmospheric methyl chloride signature (-39±~1.5 per mil). Living soils are generally significantly enriched (-28±~8 per mil). This study does not provide clear evidence of correlation between soil temperature and isotopic signature and isotopic signatures do not correlate with flux except on a chamber by chamber basis, indicating that there are multiple processes occurring simultaneously. Our results indicate that within some chambers headspace air is exchanged nearly completely within 5 minutes with the soil pore space.