Intertidal mangrove forests contribute significantly to biogeochemical solute budgets of tropical and subtropical coastlines. A significant part of the biogeochemical cycling in mangrove ecosystems ...occurs within the subsurface of the forest floor. This subsurface source and the ‘offshore’ sink are linked by the tidally-driven movement of water through the mangrove forest floor. The tidal circulation through animal macro-burrows in the Coral Creek mangrove forest (area 3km2) on Hinchinbrook Island (Australia) is documented by constructing mass balances of isotopes of radium and radon measured in the creek in 1997 and 2005 respectively. Isotope activities in burrows were found to be significantly greater than in the adjacent mangrove creek (e.g. for 222Rn, 680–1750 and 30–170Bqm−3 in burrows and creek respectively). From isotope mass balances, a daily water circulation flux through burrows of 30.4±4.7Lm−2 of forest floor is calculated. This study quantifies the underlying physical process, i.e. the tidal flushing of burrows, that supports significant and continuous exports of soluble organic and inorganic matter from mangrove forest floors to the coastal ocean. The potential significance of the circulation flux estimate is illustrated by up-scaling of the forest-scale estimates to the mangrove forests in the entire central Great Barrier Reef. This extrapolation indicates that the annual water flux circulated by this tidally-driven ‘mangrove pump’ is equivalent to as much as 20% (16–22%) of the total annual river discharge along the ca. 400km long coastline of this region.
Full text
Available for:
GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK
Estimates of post-Last Glacial Maximum (LGM) sea-level rise are not balanced by the estimated amount of ice melted since the LGM. We quantify this “missing ice” by reviewing the possible ...contributions from each of the major ice sheets. This “missing ice” amounts to 18.1 ± 9.6 m of global sea-level rise. Ocean expansion accounts for 2.4 ± 0.3 m of this discrepancy while groundwater could contribute a maximum of another 1.4 m to this offset. After accounting for these two potential contributors to the sea-level budget, the shortfall of 15.6 ± 9.6m suggests that either a large reservoir of water (e.g. a missing LGM ice sheet) has yet to be discovered or current estimates of one or more of the known LGM ice sheets are too small. Included within this latter possibility are potential inadequacies of current models of glacial isostatic adjustment.
•We quantify the amount of “missing ice” at the LGM.•The ocean-density component to sea-level rise since the LGM is 2.4 ± 0.3 m.•Groundwater contributed no more than 1.4 m to sea-level rise since the LGM.•These two components leave 15.6 ± 9.7 m of missing sources of sea-level rise since the LGM.
Full text
Available for:
GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
•Current groundwater recharge in the western US is synthesized.•Recharge components are compared across the selected aquifers.•Climate-change is analyzed to determine impact on total recharge and ...mechanism.•Geographical patterns in total recharge and mechanism changes are described.•Knowledge gaps that limit predictions of future changes in recharge are identified.
Existing studies on the impacts of climate change on groundwater recharge are either global or basin/location-specific. The global studies lack the specificity to inform decision making, while the local studies do little to clarify potential changes over large regions (major river basins, states, or groups of states), a scale often important in the development of water policy. An analysis of the potential impact of climate change on groundwater recharge across the western United States (west of 100° longitude) is presented synthesizing existing studies and applying current knowledge of recharge processes and amounts. Eight representative aquifers located across the region were evaluated. For each aquifer published recharge budget components were converted into four standard recharge mechanisms: diffuse, focused, irrigation, and mountain-systems recharge. Future changes in individual recharge mechanisms and total recharge were then estimated for each aquifer. Model-based studies of projected climate-change effects on recharge were available and utilized for half of the aquifers. For the remainder, forecasted changes in temperature and precipitation were logically propagated through each recharge mechanism producing qualitative estimates of direction of changes in recharge only (not magnitude). Several key patterns emerge from the analysis. First, the available estimates indicate average declines of 10–20% in total recharge across the southern aquifers, but with a wide range of uncertainty that includes no change. Second, the northern set of aquifers will likely incur little change to slight increases in total recharge. Third, mountain system recharge is expected to decline across much of the region due to decreased snowpack, with that impact lessening with higher elevation and latitude. Factors contributing the greatest uncertainty in the estimates include: (1) limited studies quantitatively coupling climate projections to recharge estimation methods using detailed, process-based numerical models; (2) a generally poor understanding of hydrologic flowpaths and processes in mountain systems; (3) difficulty predicting the response of focused recharge to potential changes in the frequency and intensity of extreme precipitation events; and (4) unconstrained feedbacks between climate, irrigation practices, and recharge in highly developed aquifer systems.
Full text
Available for:
GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK, ZRSKP
Fundamental approaches to the study of groundwater rely on investigating the spatial and temporal distribution of stable and radioactive isotopes and other anthropogenic compounds in natural ...waterbodies. The most often used tracers for estimating groundwater flow paths and residence times, groundwater/surface water interaction as well as tracing chemical (contamination) sources include stable isotopes of water (δ 18O and δ 2H), radiocarbon (14C; t1/2 = 5730 a), tritium (3H; t1/2 = 12.43 a) as well as unreactive fluorine-containing gases (e.g., chlorofluorocarbons CCl3F or CFC-11; CCl2F3 or CFC-12; C2Cl3F3 or CFC-113; and SF6). While gas tracers are usually referred to as transient tracers and are appropriate for investigating modern flow systems, the isotopic tracers are often used to investigated paleo or regional flow systems. Stable isotopes of water can also be used to investigate groundwater/surface water interactions. Another, thus far been less frequently used group of groundwater tracers, are cosmo- and geo- genic short-lived radioisotopes. These isotopes are uniquely suited for studying a wide range of groundwater problems that have short time scales including high aquifer vulnerability to quantitative and qualitative impacts and groundwater discharge to surface waters. Here, we discuss and compare the applications of radio‑sulphur (35S; half-life t1/2 = 87 d), radio‑beryllium (7Be; t1/2 = 53 d), radio‑phosphorus (32/33P; combined t1/2 = 33 d), natural tritium (3H; t1/2 = 12.43 a), radon (222Rn; t1/2 = 3.8 d) and short-lived radium (224/223Ra; combined t1/2 = 5.2 d). The paper discusses the principles of the individual tracer methods, focusing on the isotopes' input functions or values, on sampling techniques, and on methods of analyses. Case studies that applied a combined use of the tracers are referred to for readers who wish to learn more about the application of the so far underused cosmo- and geo- genic radioisotopes as aquatic tracers.
Display omitted
•The presence of groundwater of sub-yearly residence time indicates high aquifer vulnerability.•However, commonly used age tracers do not cover this short time range satisfactorily.•A state-of-the-art review regarding the related use of 35S, 7Be, 32/33P, 222Rn and 224/223Ra is presented.•Promising novel applications of these short-lived naturally occurring radionuclides are introduced.•Examples of multi-tracer approaches employing the set of radionuclides are discussed.
Full text
Available for:
GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
► We compile a 13-year record of groundwater age for springs in a mountain watershed. ► Annual variations in age are large and correlated with annual snowpack. ► Time-series data provide important ...constraints on age mixing models and mean ages. ► Generally upward trends in mean age suggest declining recharge. ► This work demonstrates that groundwater age monitoring can be a valuable tool.
We compile a unique 13-year record of groundwater age for 11 springs in Sagehen basin, a watershed in the Sierra Nevada Mountains of California, USA. Chlorofluorocarbon (CFC), sulfur hexafluoride (SF6), and tritium (3H) data collected in prior studies from 1997 to 2003 are re-interpreted and combined with new data collected in 2009 and 2010. The age record is analyzed to explore the potential value of groundwater age monitoring for (1) providing additional constraints on the age distribution in mixed-age samples, and (2) identifying temporal changes in groundwater recharge. Motivation for this study is provided by a lack of knowledge of how groundwater recharge and discharge (stream baseflow) in mountain watersheds might respond to climate change, and a resulting need to better understand mountain aquifer residence times, storage, and recharge. Piston-flow ages for the springs generally range from 10 to 50yr. The plausibility of different age mixing models is tested by comparing observed temporal variations in age with those simulated using simple numerical models, and by examining plots comparing the concentrations of different age tracers. We find that most spring waters are best characterized by a bimodal mixing model consisting of a new (<1yr old) fraction and a fraction that is older, but still modern (recharged after 1950). Identification of this mixing model would not have been possible without data from multiple age tracers and data from multiple years. Computed mean ages vary substantially (often by 3–7yr) between sampling events for most springs, including those with ages of 20–50yr. Mean age variations are likely controlled by variations in the magnitude of the new fraction, which is positively correlated with annual snowpack water content. Most springs show overall upward trends in mean age for the sampling period, consistent with decreasing recharge rates in response to diminishing snowpack. Groundwater age monitoring appears to be a potentially valuable tool for both constraining the age distribution in sampled waters and investigating recharge transients.
Full text
Available for:
GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK, ZRSKP
•New method for estimating hyporheic exchange based on stream radon mass balance.•Most sensitive to flow paths tens of metres long with residence times of days.•New method and injected tracer could ...capture spectrum of hyporheic exchange.
Hyporheic and parafluvial flows between streams and the underlying streambed, or adjacent alluvium, are important drivers of biogeochemical cycling in streams. Here we present a new method for characterising this exchange in a losing stream based on longitudinal stream radon activities. A mass balance approach is used to constrain the radon influx into the stream and estimate exchange parameters: flux, residence time and exchange zone thickness. A net radon flux into the stream of 5.4×104Bqm−1d−1 is required to balance radon losses to groundwater recharge, gas transfer and radioactive decay. Given the radon production rate of the sediments (1.3±0.7BqL−1d−1), the minimum volume of alluvium flushed by either hyporheic or parafluvial exchange is 168m3 per m length of stream. Based on the stream width, depth of alluvial sediments and porosity, this implies that the exchange zone extends beneath the stream and an additional 11m either side. The results of this new method are compared to two existing methods; streambed radon disequilibrium and transient storage modelling of breakthrough curves of an injected tracer. The stream radon mass balance provides a relatively simple means of estimating hyporheic (and parafluvial) exchange over tens to hundreds of kilometres of stream. Concurrent application of the stream radon method, transient storage modelling of injected tracer breakthrough curves and hydraulic methods is recommended to capture the full spectrum of hyporheic exchange in losing streams.
Full text
Available for:
GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK
Sulfur hexafluoride (SF6) is an established tracer for use in managed aquifer recharge projects. SF6 exsolves from groundwater when it encounters trapped air according to Henry's law. This results in ...its retardation relative to groundwater flow, which can help determine porous media saturation and flow dynamics. SF6 and the conservative, nonpartitioning tracer, bromide (Br− added as KBr), were introduced to recharge water infiltrated into stacked glacial aquifers in Thurston County, Washington, providing the opportunity to observe SF6 partitioning. Br−, which is assumed to travel at the same velocity as the groundwater, precedes SF6 at most monitoring wells (MWs). Average groundwater velocity in the unconfined aquifer in the study area ranges from 3.9 to 40 m/d, except in the southwestern corner where it is slower. SF6 in the shallow aquifer exhibits an average retardation factor of 2.5 ± 3.8, suggesting an air‐to‐water ratio on the order of 10−3 to 10−2 in the pore space. Notable differences in tracer arrival times at adjacent wells indicate very heterogeneous conductivity. One MW exhibits double peaks in concentrations of both tracers with different degrees of retardation for the first and second peaks. This suggests multiple flowpaths to the well with variable saturation. The confining layer between the upper two aquifers appears to allow intermittent connection between aquifers but serves as an aquitard in most areas. This study demonstrates the utility of SF6 partitioning for evaluating hydrologic conditions at prospective recharge sites.
Article impact statement: Understanding differences in tracer behaviors can inform permitting for managed aquifer recharge.
Full text
Available for:
FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SAZU, SBCE, SBMB, UL, UM, UPUK
The release of methane sequestered within deep‐sea methane hydrates is postulated as a mechanism for abrupt climate change; however, whether emitted seabed methane reaches the atmosphere is ...debatable. We observed methane emissions for a blowout from a shallow (22 m) hydrocarbon seep. The emission from the blowout was determined from atmospheric plume measurements. Simulations suggest a 1.1% gas loss to dissolution compared to ∼10% loss for a typical low‐flux bubble plume. Transfer to the atmosphere primarily was enhanced by the rapid upwelling flows induced by the massive discharge. This mechanism could allow methane suddenly released from deeper (>250 m) waters to contribute significantly to atmospheric methane budgets.
Full text
Available for:
FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SAZU, SBCE, SBMB, UL, UM, UPUK
Large quantities of natural gas are emitted from the seafloor into the stratified coastal ocean near Coal Oil Point, Santa Barbara Channel, California. Methane was quantified in the down current ...surface water at 79 stations in a 280 km2 study area. The methane plume spread over an area of ∼70 km2 and emitted on the order of 5 × 104 mol d−1 to the atmosphere. A monthly time series at 14 stations showed variable methane concentrations which were correlated with changing sub‐mesoscale surface currents. Air‐sea fluxes estimated from the time series indicate that the air‐sea flux derived for the 280 km2 area is representative of the daily mean flux from this area. Only 1% of the dissolved methane originating from Coal Oil Point enters the atmosphere within the study area. Most of it appears to be transported below the surface and oxidized by microbial activity.
Full text
Available for:
FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SAZU, SBCE, SBMB, UL, UM, UPUK
Drawing out too much groundwater, or overdrafting, is a serious problem in California. As a result, groundwater sustainability agencies are considering using recycled municipal wastewater to recharge ...aquifers. In our study, we employ suitability mapping and the models C2VSimFG and Ichnos to identify appropriate areas for managing aquifer recharge with recycled water in California's Central Valley. The factors that influence suitability include soil properties, proximity to recycled water sources, and the residence time, or amount of time that recharged water spends underground. There are many suitable areas in the Central Valley that are immediately adjacent to water recycling facilities. However, adequate supply is an issue in most locations. Roughly half of the groundwater sustainability agencies in critically overdrafted basins of the Central Valley have enough potentially suitable locations to meet their recharge goals, but not all of them have access to enough recycled water. The methods demonstrated here can serve as tools for agencies considering using recycled water for aquifer recharge.
Full text
Available for:
IZUM, KILJ, NUK, PILJ, PNG, SAZU, UL, UM, UPUK
PDF
