Isotope Tracers of Biogeochemical and Hydrologic Processes II Posters
Presiding: T Bullen, U.S. Geological Survey; J Hogan, University of Arizona
H51C-01 0830h
Quantifying Mountain Front Recharge Using Isotopic Tracers
To improve our conceptual and quantitative understanding of mountain-front/mountain-block recharge (MFR) associated with the Huachuca Mountains of the Upper San Pedro River Basin in Arizona, we employed a suite of geochemical measurements including isotopic tracers and noble gases. MFR is frequently the dominant source of recharge to alluvial basins in the semiarid Basin and Range province. It consists of mountain runoff that infiltrates at the mountain front (mountain-front recharge), and percolation through the mountain bedrock that reaches the basin via the movement of deep groundwater (mountain-block recharge). The rate of MFR can be estimated from a water balance, a Darcy's law analysis, or inverse modeling of groundwater processes. Despite the large volume of research on water resources in the basin and the critical importance of MFR to the water budget, the best estimates of MFR obtained using these methods may have errors as large as 100%. We find that geochemical tracers address mechanistic questions regarding recharge seasonality, location, and rates as well as addressing groundwater flowpaths and residence times. The gradient of stable isotopes of hydrogen and oxygen in groundwater with elevation mirrors that of regional precipitation, providing a constraint on the location and seasonality of recharge. Stable isotopic signatures indicate that MFR is dominated by winter precipitation but has 1/3 or more contribution from monsoon precipitation. Detectable tritium and 14C values greater than 100 pMC for springs, shallow groundwater in mountain canyons, and from wells along the mountain front indicate decade-scale residence times. Away from the mountain front 14C values rapidly decrease, reaching 12.3±0.2 pMC near the river. This suggests total basin residence times greater than 10,000 years, consistent with past measurements. Ongoing analysis of noble gas concentrations will provide an indication of recharge conditions. The solubility of noble gases in water depends on temperature and pressure; thus, noble gas concentrations provide a means to distinguish water samples recharged at different elevations.
H51C-02 0830h
Hydrologic Contributions of Springs to the Logan River, Utah
The Logan River flows through a fractured karst watershed of the Bear River mountain range in northern Utah, and provides significant water supply to the city of Logan, Utah. Springs flowing into the Logan River are important sources of water after annual snowmelt has been exhausted. In this work, we present results from a year of monitoring water chemistry and stable isotopes (D, 18O, and 13C) in two major springs and in the Logan River upstream and downstream of the combined spring inputs. The two springs, DeWitt and Spring Hollow, flow into the river within 1.5 km of each other. Annual patterns of Si and Mg suggest a flushing pattern, with reduced concentrations during snowmelt, and increasing concentrations throughout baseflow recession, at all for sampling locations. Cl concentrations are likewise greatly depressed after the snowmelt pulse but afterward remain consistently low at all four sites. Stable isotope data show that spring water is generally more enriched in D and 18O than river water, with an enriching pattern throughout annual stream flow recession.
http://www.mines.edu/~mgooseff/web_research/loganriver.html
H51C-03 0830h
Nitrogen Isotopes - Tracers for Nitrogen Transformations in Small Constructed Wetlands
Small artificial wetlands are increasingly applied to reduce non-point source pollution and to contribute to the restoration of eutrophicated freshwaters and coastal marine ecosystems. In the agricultural catchment area of Lake Sempach in Central Switzerland, several wetlands were constructed at the interface between subsurface drainage system and receiving waters. In order to evaluate nitrogen retention efficiency of the systems and to identify major processes controlling their performance, nitrate, ammonium, particulate and dissolved organic nitrogen in the inflowing and outflowing water as well as Δ15N of nitrate, ammonium and sediment were analyzed in a three pond system. During the studied 26 months the wetland retained 27% of the nitrogen load corresponding to a surface related retention of 51g N m-2 yr-1. Nitrate in the inlet contributed 48% to the total nitrogen load and was characterized by high Δ15N-NO3(>13‰) typical for intensively manured grassland soils. Increasing Δ15N-NO3 in the wetland compared to the inlet was attributed to denitrification whereas decreasing Δ15N-NO3 during stagnant periods was due to anoxic mineralization of particulate organic matter and subsequent nitrification.
H51C-04 0830h
Stable Isotopic Signatures of Soil Nitrogen and Carbon Across Forty Years of Forest Development
The Δ15N and Δ13C signatures of soil organic matter provide integrative measures of ecosystem-level N and C processes. Current understanding of the extent to which various mechanisms govern soil Δ15N and Δ13C is limited by the complexity of processes occurring simultaneously in soils, and the notable absence of Δ15N and Δ13C data from individual sites throughout the ecosystem's development. We examined Δ15N and Δ13C and associated data of archived soil samples from four depths in an aggrading loblolly pine forest in the southeastern US. We examine these data in conjunction with O horizon, root, litterfall, and foliage isotopic data, as well as with previously published information about soils at this site, and identify mechanisms driving these parameters' observed shifts with time. Soil Δ15N increased with depth, and across time in the three deepest layers, by a maximum of 5.5‰, to 9.1‰. We isolate two key mechanisms that have governed changes in N isotopes in these soils. First, though discrimination against 15N during SOM mineralization is relatively small compared to other processes in the N cycle, the mineralization and transfer of >800 kg ha-1 soil organic N into aggrading vegetation and forest floor suggest that isotopic fractionation associated with SON mineralization has been a governing feature of these soils' Δ15N values. Second, accretion of microbial residues enriched in 15N likely has influenced soil Δ15N via microbial excretion of 15N-deplete compounds. We suggest that the greater N isotope fractionation associated with microbial dissimilation of N compounds compared to the fractionation associated with N assimilation results in a significant enrichment of microbial biomass and subsequent residues. Though most chemical reactions in the N cycle result in isotopic fractionation, several of these processes are likely insignificant in these soils, permitting us to isolate these two likely influences on soil Δ15N. Soil Δ13C increased with depth, and declined by 1.5‰ during forest development in the surface layer. Deeper layers exhibited no significant trend in Δ13C with time. Because these soils periodically supported C4 vegetation prior to forest planting, the initial Δ13C values were more enriched than pure C3 vegetation (-24.5‰). We expect that as C3 plant residues become more fully incorporated into the soil profile, an increasing proportion of SOM comprised of microbial residues will promote an increase in Δ13CSOM, particularly deeper in the soil profile.
H51C-05 0830h
Evaluating nitrate sources in nested agricultural sub-basins using nitrate stable isotopes
Nutrient enrichment is the second leading cause of drinking water contamination in the United States. To provide environmental managers with nutrient source and transport information, the U.S. Geological Survey' s National Water-Quality Assessment (NAWQA) Program conducted a multi-component study in Sugar Creek Basin, Indiana, in which major nutrients, cations, anions, and pesticides were analyzed. Land use at Sugar Creek (246 square km basin) is dominated by row crop agriculture, primarily corn and soybeans. The soils are largely heavy clay, glacial till in origin, and require tile drains to move excess water and make the land farmable. As one component of the study, stable isotopes of nitrate (N-15 and O-18) were used to examine nitrate sources and transport, and possible transformations of nitrate. Water samples were collected in 2003 and 2004 from major environmental compartments involved with the movement of nutrients into the creek, (precipitation, tile drain, and overland flow). Samples were also collected from Leary-Weber Ditch, a 6.2 square km basin is nested within Sugar Creek. Collection times bracketing four distinct periods of the agricultural cycle: pre-application of fertilizer, post-application of fertilizer, growing season, and post-harvest periods. Nutrient samples (nitrate, phosphate, ammonia) were also collected several times between storm events during baseflow conditions. Preliminary nutrient and pesticide data indicate that tile drains are the primary pathway into streams. Little interaction occurs between the ground water and surface water interface. Nitrate stable isotopes will enable us to determine the relative contribution of nitrate sources feeding in from the tile drains, into Leary-Weber Ditch and Sugar Creek.
H51C-06 0830h
Tracing Causes of Hypoxia in the San Joaquin River Using Isotopic Techniques
Fish migration through the deep-water shipping channel in the San Joaquin River near the city of Stockton CA is inhibited by periodic low dissolved oxygen (DO) concentrations during low flow conditions. There is considerable controversy regarding the relative roles of two mechanisms that can contribute to DO depletion: decomposition of algae from upstream locations and nitrification of ammonium from a nearby waste water treatment facility. Development of a successful remediation plan requires knowledge of the controls on spatial and temporal differences in oxygen-consuming mechanisms. To better understand the timing and relative importance of the mechanisms responsible for oxygen depletion, samples were collected for isotopic and chemical analysis during two intensive two-day sampling trips in August 2004. Samples were taken from a stationary houseboat in the channel, and from upstream and downstream traveling boats. Water samples at the houseboat were collected at five depths at 2-4 h intervals, and samples from 1 m were collected at about 4 h intervals from the traveling boats. All samples were analyzed for DO-d18O, seston-d15N/d13C, nitrate-d15N/d18O, DIC-d13C, water-d18O/d2H, DO, ammonium, and nitrate concentrations. Of all the measured parameters, ammonium, DO, and DO-d18O showed the strongest diurnal fluctuations, as well as significant changes with depth. Physico-chemical parameters indicated diurnal stratification and overturn of the channel. The general increase in the DO-d18O coincident with decreases in DO suggests that the night-time decrease in DO is caused largely by O2 consumption, either by respiration of organic matter or by nitrification. The DIC-d13C and nitrate-d15N data indicate that nitrification may affect DO concentrations as much or more than respiration. Preliminary principle components analysis indicates that photosynthesis is the main control over DO concentrations during this period of DO depletion, and that both nitrification and respiration are significant causes of DO depletion in this channel. Future work will focus on the transition between normal DO conditions and periods of DO depletion.
H51C-07 0830h
Temporal Isotopic Variations of Leaf Water in Pine Needles
Understanding the isotopic variations in a plant's leaf water is important for a number of climatological and biogeochemical studies. Leaf water isotopic composition is affected by the isotopic composition of the source water and the relative humidity of the air, both of which are related to climate. This dependency is the basis for climate reconstruction using isotopic compositions of tree-ring cellulose. The isotopic composition of leaf water is also important for the assessment of terrestrial biological productivity and the quantification of the Dole effect. We have studied the oxygen and hydrogen isotopic variations in leaf water of biennial needles from red pine (Pinus resinosa) and white pine (Pinus strobes) in Hanover, New Hampshire, USA. We have examined the leaf water ΔD and Δ18O values along pine needles from base to tip, and the isotopic differences between young and old leaves. Within a needle, progressive enrichments of both oxygen-18 and deuterium were observed toward the tip, ranging for ΔD from -60.1 to 9.4 permil in white pine and -67.1 to -34.9 permil in red pine, and for Δ18O from -3.1 to 19.1 for white pine and -7.3 to 5.5 permil in red pine. For both species, ΔD and Δ18O were higher in old leaves than in young leaves. The isotopic difference between old and young leaves was most pronounced earlier in the growing season; the gap narrowed with time and finally disappeared in early fall. Early in the growing season, the ΔD values of young needles were -21 and -30 permil in white and red pine, respectively, and that of old needles were -3.0 and -8.0 permil, respectively. The Δ18O values showed similar trends, and the ΔD vs. Δ18O slope for the young leaves decreased from 3.6 in spring to ~1 in early autumn. Our observations can be simulated using the progressive isotopic enrichment model proposed by Barnes and Farquhar for monocotyledoneous leaves. Two variables, the transpiration rate and length of the needle, can explain the observed isotopic variations. These two variables can be combined into one parameter in the model as the longitudinal Peclet number of the leaf. In addition, the model can also explain the change in the slope of the ΔD vs. Δ18O relationship in leaf water.
H51C-08 0830h
Genetic Effect on Carbon-Isotope Composition of a Plant
Stable carbon isotopes of organic sediments are potential tools in investigating a wide spectrum of geological problems. These include paleoclimate, paleoecology, and the origin of life. The quality of the information the tools provides depends largely on our knowledge on the factors determining the isotopic composition of a plant. This is because most biogenic organic sediments are derived from plants. The factors can be grouped into internal and external. The internal factors are ultimately attributable to the genetic make-up of a plant. The most well known internal factor is the photosynthetic pathway. Others include structure of the leave tissue and metabolic characteristics of a plant. External factors are concentration and the isotopic composition of the source CO2 and the physical and chemical conditions of the plant's growth environments. This study addresses primarily the genetic effect, the internal factors. Based on the results of two suites of natural plant samples, it is concluded that the difference in photosynthetic pathway entails about 20.0 % of spread in terms of 13CPDB values. Genetic effect is also accountable for up to 7.0 to 8.0 % spread in 13CPDB values within a single category of photosynthetic pathway (i.e. the Calvin cycle). With constrains from the relevant known knowledge, it is concluded that the 13CPDB values of terrestrial plants are probably ranging from - 8.0 to equal or less than -44.9 %. This range of 13CPDB values may also be considered the bio-signature of organic sediments of great antiquity.
H51C-09 0830h
Fine-Scale Temporal Resolution of Sediment Source by Be-7
Understanding of erosional processes occurring at fine scales (cm) and over short time periods (min) in agricultural settings is essential for efforts to minimize landscape scarring, conserve surface nutrients, and reduce off-site impacts. Cosmogenic and fallout radionuclides have been successfully used in a variety of settings to determine sediment source and sediment transport processes. In this study we used the short-lived radionuclide Be-7 (t1/2= 53 d) to investigate erosional processes occurring during runoff from a 4 m by 9 m erosion plot. The plot was established in a 9.8% slope no-till corn field at the USDA ARS Deep Loess Research Station in Treynor, Iowa. Before and after the rainfall, fine resolution soil profiles were collected to determine the distribution of radionuclides and soil nutrients with depth. Be-7 was concentrated near the soil surface. Prior to the rainfall event, rare earth tagged soil particles were applied in three discrete strips, 0.5 m wide, along the contour. Forty runoff samples were collected during the course of a 5.7 cm thunderstorm event. Runoff efficiency was 25% and sediment yield was 0.234 kg m-2. Be-7 activities in runoff varied with hydrologic conditions and rainfall intensity, ranging from 0.06-0.6 Bq gm-1. Dominant erosional processes observed were rain splash erosion, overland flow and rill transport. Be-7 rich sediment was delivered at times corresponding corresponded to peaks in rainfall intensity, onset of overland flow, and development of hydrologic connectivity. Sediment had lower Be-7 activity during peak sediment delivery, probably due to dilution by large volumes of Be-7 poor sediment derived from deeper rill erosion. Soil tagged with the rare earth elements Ho, Tb, and Eu showed downslope movement in interrill areas, supporting conclusion of rain splash and sheetflow erosive mechanisms.
H51C-10 0830h
Particle Dynamics in the Temporary Deposition Zone of Lake Superior: Insights From 210Pb,137Cs and 7Be
With the aim of understanding particle dynamics in the slope-profundal region in coastal areas of the Great Lakes and the particulate organic carbon availability to benthic organisms, sediment cores were taken along three transects in the nearshore region of Lake Superior. Inventories of 137Cs and excess 210Pb and focusing factors (based on both isotopes) were calculated for 28 cores. 7Be was only found in the fluff layer of the sediments at most sites, but its presence in deep-water sites attests to the rapidity of sedimentation. The 210Pb-derived focusing factors ranged from 0.04 to 2.3 with mean and median values of 0.36 and 0.15, respectively. The study region is thus categorized as a temporary deposition zone. A 2-dimensional steady-state nested-box model was developed and applied to individual transects with the 210Pb inventories as model input. Model output included isotope residence times and cross-margin fluxes of sediments and isotopes. The time scale for the particles focusing from the shelf zone to the profundal zone of the transect was on the order of 10 years. The predicted residence times also indicate the existence of local enrichment zones for both isotopes and sediments as well as depletion zones along each transect. The results suggest that food availability may explain the observed Diporeia distributions along the transects. High ratios of 137Cs:210Pb were interpreted to indicate longshore transport by the Keweenaw Current and the general counter-clockwise circulation in the lake. This circulation entrains older sediments into the nearshore zone where they remain for periods of 10-30 years.