Pradeep K. Aggarwal

Isotope studies in large river basins: A new global research focus

Rivers are an important linkage in the global hydrological cycle, returning about 35%of continental precipitation to the oceans. Rivers are also the most important source of water for human use. Much of the worlds population lives along large rivers, relying on them for trade, transportation, industry, agriculture, and domestic water supplies. The resulting pressure has led to the extreme regulation of some river systems, and often a degradation of water quantity and quality For sustainable management of water supply agriculture, flood-drought cycles, and ecosystem and human health, there is a basic need for improving the scientific understanding of water cycling processes in river basins, and the ability to detect and predict impacts of climate change and water resources development.

Comment on "Arsenic Mobility and Groundwater Extraction in Bangladesh" (I)

Harvey et al. (1) recently presented detailed vertical profiles for groundwater arsenic and a suite of other water and sediment properties from a study site in southern Bangladesh. This information, supplemented with carbon isotopic data (C and C) for dissolved organic carbon (DOC) and dissolved inorganic carbon (DIC), was used to support the notion that the DOC introduced into subsurface aquifers with groundwater recharge could lead to reduction or dissolution of iron oxyhydroxides and the subsequent release of associated arsenic into groundwater. This is an important issue for tens of millions of people in Bangladesh and other South Asian countries, who are at risk of contracting various cancers from drinking water with elevated arsenic levels (2). The process of arsenic mobilization could be much more dynamic than previously thought if, as Harvey et al. have proposed (1), the leading agent for arsenic release from iron oxyhydroxides is indeed DOC penetrating sandy aquifers on time scales of tens of years rather than particulate organic carbon deposited over several thousand years throughout the Ganges-Brahmaputra-Meghna delta (3, 4). On the basis of particulate and dissolved sulfur data, Harvey et al. also have effectively laid to rest the argument that the oxidation and dissolution of particulate sulfide is the dominant cause of elevated arsenic in groundwater, at least for their study site. We are troubled, however, by the argument that groundwater pumping for irrigation increased the penetration of DOC into the subsurface and therefore caused the release of arsenic into the groundwater. We believe that the data in (1) and related information from other regions of Bangladesh could be equally well explained without invoking a response of groundwater arsenic to irrigation pumping. This is an important point, because policy-makers in Bangladesh and elsewhere should not be confronted with the false dilemma of elevated arsenic in groundwater caused by irrigation or insufficient agricultural production. The Harvey et al. data appear to be inconsistent with recent mobilization of arsenic by irrigation pumping, if their model of vertical flow is taken at face value. As Harvey et al. point out, the concentration of radiocarbon in the atmosphere increased to unprecedented levels starting in the 1950s because of atmospheric testing of nuclear bombs (5). The penetration of radiocarbon above pre-testing levels in subsurface aquifers is an indicator of groundwater recharge over the past 50 years. The onset of this particular anthropogenic perturbation therefore preceded the onset of massive irrigation in Bangladesh by about 25 years. According to their vertical flow model, this would mean bomb-produced C should have penetrated at least to the depth of maximum arsenic mobilization if this feature was indeed caused by an enhanced supply of DOC linked to irrigation. Yet, according to figures 1A and 3 in (1), the maximum in arsenic mobilization is observed at about 40 m depth, while only the two shallowest radiocarbon samples, at 3 and 19 m, indicate addition of bomb radiocarbon. Thus, the Harvey et al. data do not necessarily indicate a direct connection between arsenic mobilization and increased irrigation pumping in their study area. Even if the portion of the Harvey et al. argument that we dispute were correct in their study area, there is convincing evidence that it does not apply to other parts of Bangladesh. Tritium (H) is another radionuclide whose concentration in the atmosphere increased dramatically in response to atmospheric bomb testing (6, 7). The distribution of H in water has therefore also been used extensively to study oceanic circulation as well as groundwater recharge. A recent report from the International Atomic Energy Agency (8) lists eight samples with significantly elevated arsenic levels that do not contain any detectable H. We have collected and analyzed an additional 49 groundwater samples from other wells in Bangladesh that include a set of 5 paired arsenic and H analyses indicating high arsenic levels without detectable H (9, 10). The implication of the combined data set is that over a dozen carefully analyzed samples indicate that Bangladesh groundwater was elevated in arsenic well before the onset of massive irrigation. We therefore believe that increased irrigation over the past 25 years is unlikely to have caused widespread arsenic mobilization in Bangladesh groundwater through the sequence of steps proposed by Harvey et al. in their otherwise very valuable contribution.

Maps and animations offer new opportunities for studying the global water cycle

The International Atomic Energy Agency/World Meteorological Organization Global Network for Isotopes in Precipitation (IAEA/WMO GNIP) data base includes more than 100,000 δ18O,δ2H, and 3H measurements performed on monthly precipitation samples collected at 550 stations worldwide. Since 1961, the data base has served as a baseline reference for the distribution of water isotopes in modern precipitation. It is widely used in the fields of isotope hydrology, climatology oceanography and paleoclimatology.

Approaches for Achieving Long-Term Accuracy and Precision of δ18O and δ2H for Waters Analyzed using Laser Absorption Spectrometers

The measurement of δ(2)H and δ(18)O in water samples by laser absorption spectroscopy (LAS) are adopted increasingly in hydrologic and environmental studies. Although LAS instrumentation is easy to use, its incorporation into laboratory operations is not as easy, owing to extensive offline data manipulation required for outlier detection, derivation and application of algorithms to correct for between-sample memory, correcting for linear and nonlinear instrumental drift, VSMOW-SLAP scale normalization, and in maintaining long-term QA/QC audits. Here we propose a series of standardized water-isotope LAS performance tests and routine sample analysis templates, recommended procedural guidelines, and new data processing software (LIMS for Lasers) that altogether enables new and current LAS users to achieve and sustain long-term δ(2)H and δ(18)O accuracy and precision for these important isotopic assays.

Global network is launched to monitor isotopes in rivers

The Global Network of Isotopes in Rivers (GNIR), launched by the International Atomic Energy Agency (IAEA) in 2007, compiles water isotope data on rivers to complement the 45-year-old IAEA/World Meteorological Organization (WMO) global network of isotopes in precipitation (GNIP). River runoff carries an integrated memory of hydrological processes in a basin. Recent studies [e.g., Vorosmarty and Meybeck, 2004] suggest that the impacts of storages, diversions, and redirection of streamflow for water supply, hydropower, and irrigation might surpass the impact of recent and anticipated future climate changes on river runoff. Consequences of these effects include changes in the frequency and extent of flooding, increased sediment load, altered groundwater recharge, and degradation of water quality and riparian ecosystems. These consequences often result in political disputes or upstream-downstream inequities. GNIR aims to provide an improved understanding of stream/aquifer interactions, the impacts of climate changes on river runoff, and especially human impacts on river discharge with the use of isotope data.

A Review of Isotope Applications in Catchment Hydrology

Isotope methods were introduced into catchment hydrology research in the 1960s as complementary tools to conventional hydrologic methods for addressing questions of where water goes when it rains, what pathways it takes to the stream and how long water resides in the catchment (McDonnell, 2003). Despite slow incorporation into routine research applications, the last decade has seen a rapid increase in isotope-based catchment studies. These have been mainly carried out in small well-instrumented experimental catchments, on the order of 0.01 to 100 km and located typically in headwater areas (Buttle, 1998). In contrast, little has been done in terms of application and transfer of these concepts and methodologies to large (>100s to 1000s of km), less instrumented basins. Much potential also waits to be realized in terms of how isotope information may be used to calibrate and test distributed rainfall-runoff models and to aid in the quantification of sustainable water resources management. In this chapter, we review the major applications of isotopes to catchment studies, and address a variety of prospective new directions in research and practice. Our discussion is based primarily on catchments in temperate to wet zones.

Stable Isotope Composition of Molecular Oxygen in Soil Gas and Groundwater: A Potentially Robust Tracer for Diffusion and Oxygen Consumption Processes

Abstract We have measured the concentration and isotopic composition of molecular oxygen in soil gas and groundwater. At a site near Lincoln, Nebraska, USA, soil gas oxygen concentrations ranged from 13.8 to 17.6% at depths of 3–4 m and the δ18O values ranged mostly from 24.0 to 27.2‰ (SMOW). The concentration of dissolved oxygen in a perched aquifer in the Texas Panhandle (depth to water ∼76 m) was about 5 mg/L and the δ18O values were 21.2–22.9‰. The δ18O of soil gas oxygen in our study are higher and those of dissolved oxygen are lower than the δ18O of atmospheric oxygen (23.5‰). A model for the oxygen concentration and isotopic composition in soil gas was developed using the molecular diffusion theory. The higher δ18O values in soil gas at the Nebraska site can be explained by the effects of diffusion and soil respiration (plant root and bacterial) on the isotopic composition of molecular oxygen. The lower δ18O of dissolved oxygen at the Texas site indicates that oxygen consumption below the root zone in the relatively thick unsaturated zone here may have occurred with a different fractionation factor (either due to inorganic consumption or due to low respiration rates) than that observed for the dominant pathways of plant root and bacterial respiration. It is concluded that the use of the concentration and isotopic composition of soil gas and dissolved oxygen should provide a robust tool for studying the subsurface gaseous diffusion and oxygen consumption processes.

NETPATH-WIN: An Interactive User Version of the Mass-Balance Model, NETPATH

NETPATH-WIN is an interactive user version of NETPATH, an inverse geochemical modeling code used to find mass-balance reaction models that are consistent with the observed chemical and isotopic composition of waters from aquatic systems. NETPATH-WIN was constructed to migrate NETPATH applications into the Microsoft WINDOWS® environment. The new version facilitates model utilization by eliminating difficulties in data preparation and results analysis of the DOS version of NETPATH, while preserving all of the capabilities of the original version. Through example applications, the note describes some of the features of NETPATH-WIN as applied to adjustment of radiocarbon data for geochemical reactions in groundwater systems.

Tritium in Japanese precipitation following the March 2011 Fukushima Daiichi Nuclear Plant accident

Tritium concentrations in Japanese precipitation samples collected after the March 2011 accident at the Fukushima Dai-ichi Nuclear Power Plant (FNPP1) were measured. Values exceeding the pre-accident background were detected at three out of seven localities (Tsukuba, Kashiwa and Hongo) southwest of the FNPP1 at distances varying between 170 and 220 km from the source. The highest tritium content was found in the first rainfall in Tsukuba after the accident; however concentrations were 500 times less than the regulatory limit for tritium in drinking water. Tritium concentrations decreased steadily and rapidly with time, becoming indistinguishable from the pre-accident values within five weeks. The atmospheric tritium activities in the vicinity of the FNPP1 during the earliest stage of the accident was estimated to be 1.5×10(3) Bq/m(3), which is potentially capable of producing rainwater exceeding the regulatory limit, but only in the immediate vicinity of the source.

Lower Groundwater 14C Age by Atmospheric CO2 Uptake During Sampling and Analysis

Uptake of atmospheric CO₂ during sample collection and analysis, and consequent lowering of estimated ages, has rarely been considered in radiocarbon dating of groundwater. Using field and laboratory experiments, we show that atmospheric CO₂ can be easily and rapidly absorbed in hyperalkaline solutions used for the extraction of dissolved inorganic carbon, resulting in elevated ¹⁴C measurements. Kinetic isotope fractionation during atmospheric CO₂ uptake may also result in decrease of δ¹³C, leading to insufficient corrections for addition of dead carbon by geochemical processes. Consequently, measured ¹⁴C values of groundwater should not be used for age estimation without corresponding δ¹³C values, and historical ¹⁴C data in the range of 1 to 10% modern Carbon should be re-evaluated to ensure that samples with atmospheric contamination are recognized appropriately. We recommend that samples for ¹⁴C analysis should be collected and processed in the field and the laboratory without exposure to the atmosphere. These precautions are considered necessary even if ¹⁴C measurements are made with an accelerator mass spectrometer.