The water balance of a small lake using stable isotopes and tritium

A small maar lake, known as the Blue Lake, set in a karstic region of southeastern Australia, provides the municipal water supply for a population of 20,000. The lake has a volume of 3.6·10⁷m³, of which 10–15% is pumped from it each year. The lake is recharged almost entirely from groundwater and the main objective of this study was to estimate the total groundwater inflow and outflow rates. Estimation of groundwater throughflow in a lake is difficult to assess using classical hydrological techniques and an alternative method involving measurement of the concentrations of the environmental isotopes ³H, ¹⁸O, ²H and ¹⁴C in the lake water and in the recharging groundwater was used to establish the lake-water balance. The water-balance calculations indicated a total groundwater inflow to Blue Lake of between 5.0·10⁶ and 6.5·10⁶m³yr.⁻¹, corresponding to a residence time of water in the lake of 6 yr. It was not possible to derive the relative proportions of inflow to the lake from the two possible source aquifers, using these isotopes, because their concentrations did not show a sufficiently large contrast to distinguish the two water sources.     

High concentration of solutes at the upper part of the saturated zone (water table) of a deep aquifer under sewage-irrigated land

The water-table region (upper 50 cm of the saturated zone) of a 25 m deep phreatic sandstone aquifer, lying under fields irrigated with sewage effluents for up to 22 yrs, was monitored in 1971 and 1984. Average concentrations of NO⁻₃, Cl⁻ and SO²⁻₄ of up to 225, 307 and 155 mg l⁻¹, respectively, were detected in the upper 50 cm of the saturated region in two research wells in 1984. These concentrations, which are related to effluent and fertilizer input to groundwater, were two to four times higher than those found deep (37–55 m) below the water table in nearby (1000 m distant) production wells. Nitrate data and the estimated transit time through the unsaturated zone (2 m yr⁻¹) support the model suggesting that the major source of nitrate pollution in the past should be related to the oxidation of soil organic matter. The SO²⁻₄/Cl⁻ ratio is found to be a useful indicator for the arrival of SO²⁻₄-fertilizers at the groundwater interface. The observations presented in this paper question the suitability of plans for using effluents as a water source for agriculture in regions which are the replenishment areas of phreatic aquifers.     

PSYCHIC – A process-based model of phosphorus and sediment transfers within agricultural catchments. Part 2. A preliminary evaluation

This paper describes the preliminary evaluation of the PSYCHIC catchment scale (Tier 1) model for predicting the mobilisation and delivery of phosphorus (P) and suspended sediment (SS) in the Hampshire Avon (1715 km²) and Herefordshire Wye (4017 km²) drainage basins, in the UK, using empirical data. Phosphorus and SS transfers to watercourses in the Wye were predicted to be greater than corresponding delivery in the Avon; SS, 249 vs 33 kg ha⁻¹ yr⁻¹; DP, 2.57 vs 1.26 kg ha⁻¹ yr⁻¹; PP, 2.20 vs 0.56 kg ha⁻¹ yr⁻¹. The spatial pattern of the predicted transfers was relatively uniform across the Wye drainage basin, whilst in the Avon, delivery to watercourses was largely confined to the river corridors and small areas of drained land. Statistical performance in relation to predicted exports of P and SS, using criteria for relative error (RE) and root mean square error (RMSE), reflected the potential shortcomings associated with using longer-term climate data for predicting shorter-term (2002–2004) catchment response and the need to refine calculations of point source contributions and to incorporate additional river basin processes such as channel bank erosion and in-stream geochemical processing. PSYCHIC is therefore best suited to characterising longer-term catchment response.     

Influence of artificial groundwater lakes on the abundance and activity of bacteria in adjacent subsurface systems

Bacterial abundances and activity, estimated by 4′,6-diamidino-2-phenylindole staining (DAPI) and the reduction of 2-(p-iodophenyl)-3-(p-nitrophenyl)-5-phenyl tetrazolium chloride (INT), were investigated in two oligotrophic artificial groundwater lakes and the surrounding aquifers. To evaluate the effect of lake water on groundwater downstream, samples were taken from wells at different distances from the lakes, and the total number of bacteria and the number of active bacteria in these samples were compared with samples collected upstream. In addition, sterilized sandy sediments were exposed in groundwater wells to measure the number and activity of bacteria attached to particles. At one of the study sites, where the lake sediments were disturbed by dredging, total bacterial abundance and the number of respiring bacteria in the groundwater aquifer was clearly influenced by the lake water. The average bacterial abundances decreased from 2.6 ± 1.9 × 10⁵ cells ml⁻¹ in the well closest to the lake (S2) to 2.9 ± 3.8 × 10⁴ cells ml⁻¹ in the most distant one (S4), which was equivalent to cell numbers in the upstream well. The number of respiring bacteria showed a similar tendency with 1.3 ± 2.7 × 10⁴ active cells ml⁻¹ in S2 and 1.9 ± 1.5 × 10³ active cells ml⁻¹ in S4. At the second study site, which was not influenced by dredging, bacteria in the downstream wells seemed not to be affected by the lake water. The number and activity of bacteria, which colonized exposed sediments, were not significantly different in the upstream and downstream wells, indicating a minor influence of lake water on this habitat. Our results suggest that gravel-pit lakes may influence the free living bacterial assemblages in nearshore groundwater systems, but do not visibly affect numbers and activity of bacteria attached to the surface of aquifer sediments.     

Hydraulic and geochemical interactions between surface water and sediment pore water in seasonal hypersaline Maharlu Lake,Iran

Study of interactions between surface-water and pore-water in lakes is complicated due to spatio-temporal heterogeneities in flow condition across the sediment–water interface. In this study, seasonal hypersaline Maharlu Lake was investigated by collecting surface-water and pore-water samples from four nests of multilevel piezometers installed at different distances from the inflow of rivers to the lake. The hydraulic heads in the piezometers as well as vertical profiles of Mg⁺², Na/Cl, and Br/Cl were used to investigate both hydraulic and geochemical interactions between surface-water and pore-water in the lake. Depletion of lake surface water and pore water with respect to B, Br, Li⁺, K⁺, Mg²⁺ and the absence of Mg-K chlorides and sulphates in the lake bed sediments is probably due to leakage of highly evaporated residual brine from the lake. Hydraulic gradients in the multilevel piezometric nests indicate that a general downward flow from surface-water to pore-water occurs across sediment–water interface. Vertical profiles of Br/Cl, Mg²⁺, and Na/Cl showed that the maximum flow rate was more than 1 m/yr close to the mouth of the inflowing rivers. The downward vertical flow was limited in the area far from the inflowing rivers due to the presence of an impermeable confining halite layer which interrupts the hydraulic connection between shallow pore water (less than 50 cm deep) and deeper zones. The hydraulic and geochemical interactions between surface-water and pore-water across sediment–water interface in the Maharlu Lake are of interest to find out the fate of pollutants and their distribution in the lake.     

Modeling decadal timescale interactions between surface water and ground water in the central Everglades, Florida, USA

Surface-water and ground-water flow are coupled in the central Everglades, although the remoteness of this system has hindered many previous attempts to quantify interactions between surface water and ground water. We modeled flow through a 43,000 ha basin in the central Everglades called Water Conservation Area 2A. The purpose of the model was to quantify recharge and discharge in the basin's vast interior areas. The presence and distribution of tritium in ground water was the principal constraint on the modeling, based on measurements in 25 research wells ranging in depth from 2 to 37 m. In addition to average characteristics of surface-water flow, the model parameters included depth of the layer of ‘interactive’ ground water that is actively exchanged with surface water, average residence time of interactive ground water, and the associated recharge and discharge fluxes across the wetland ground surface. Results indicated that only a relatively thin (8 m) layer of the 60 m deep surfical aquifer actively exchanges surface water and ground water on a decadal timescale. The calculated storage depth of interactive ground water was 3.1 m after adjustment for the porosity of peat and sandy limestone. Modeling of the tritium data yielded an average residence time of 90 years in interactive ground water, with associated recharge and discharge fluxes equal to 0.01 cm d⁻¹. ³H/³He isotopic ratio measurements (which correct for effects of vertical mixing in the aquifer with deeper, tritium-dead water) were available from several wells, and these indicated an average residence time of 25 years, suggesting that residence time was overestimated using tritium measurements alone. Indeed, both residence time and storage depth would be expected to be overestimated due to vertical mixing. The estimate of recharge and discharge (0.01 cm d⁻¹) that resulted from tritium modeling therefore is still considered reliable, because the ratio of residence time and storage depth (used to calculated recharge and discharge) is much less sensitive to vertical mixing compared with residence time alone. We conclude that a small but potentially significant component of flow through the Everglades is recharged to the aquifer and stored there for years to decades before discharged back to surface water. Long-term storage of water and solutes in the ground-water system beneath the wetlands has implications for restoration of Everglades water quality.     

Constraints on hydraulic parameters and implications for groundwater flux across the upland–estuary interface

Analyses of independent laboratory- and field-scale measurements from two sites on Sapelo Island, Georgia reveal heterogeneity in hydraulic parameters across the upland–estuary interface. Regardless of the method used (short-duration pumping tests, amplitude attenuation of tidal pumping data, sediment grain size distributions, and falling head permeameter tests), we obtain hydraulic conductivity of 10⁻⁴ m s⁻¹ for the fine-grained, well-sorted, clean sands that make up the upland areas. Proximal to the upland–estuary boundary, the tidal pumping analyses and permeameter tests suggest that hydraulic conductivities decrease by more than two orders of magnitude, a result consistent with the presence of a clogging layer. Such a clogging layer may arise due to a variety of physical, chemical, or biological processes. The extent and orientation of the layers of reduced hydraulic conductivity near the upland–estuary boundary influence the nature of the aquifer's response to tidal forcing. Where the lower conductivity layer forms a relatively flat creek bank, tidal pumping produces a primarily mechanical response in the adjacent aquifer. Where the creek bank is nearly vertical, there is a more direct hydraulic connection between the tidal creek and the adjacent aquifer. The clogging layer likely contributes to the development of complicated flow pathways across the upland–estuary boundary. Effective flow paths calculated from tidal pumping data terminate within the marsh, beyond the boundary of the upland aquifer, suggesting a diffuse regime of groundwater discharge in the marsh. We postulate that, in many settings, submarsh flow may be as important as seepage faces for groundwater discharge into the marsh–estuary complex.     

Formation of a salt plume in the Coastal Plain aquifer of Israel: the Be''er Toviyya region

The formation and development of a salt plume (salinity up to 800 mg Cl 1⁻¹) in the inner part of the Coastal Plain aquifer of Israel is analyzed. Massive groundwater exploitation during the 1950s caused a large drop in the water level and formation of a hydrologic depression in the Be'er Toviyya-Kefar Warburg area. The depression reached a maximal depth during the late 1960s; thereafter a reduction in the rate of pumpage led to restoration of water levels and shallowing of the depression, until its complete disappearance towards the end of the 1980s. A spot of high salinity first appeared in 1956, following a deep drawdown in the water levels. This saline plume has been continuously expanding with increasing salinity concentrations (200–800 mg Cl 1⁻¹) in its center. The average rate of radial expansion was about 50 m year⁻¹. The expansion and salinization did not cease as the depression disappeared. Rather, equalization of water levels in wells situated within the plume area with those of situated along its margins resulted in the salinization of the latter within a period of 1 year.

Mass balances for water and chloride contents were made for the period 1967–1990. Taking into consideration the storage change, pumpage, natural replenishment and artificial recharge, the lateral inflow to the depression is estimated as 60 × 10⁶ m³. Upon addition of the chloride balance, and taking into consideration the chloride concentrations of the surrounding fresh water and the apparent possible end-member of the saline source (based on geochemical considerations), the saline inflow is estimated as (40–60) × 10⁶ m³. These estimates indicate that a large amount of saline water penetrated into the aquifer, of about half of the natural replenishment of the study area, with an estimated salinity of 1900–2700 mg Cl 1⁻¹.

It is suggested that the salt plume was formed as a result of a drop in water level combined with a flow of underlying saline water bodies from deeper strata. The chemical composition of the groundwater points to the existence of two saline water bodies of Ca-chloride composition and a marine Br/Cl ratio: (1) saline water with low Na/Cl (0.6), So₄/Cl, and B/Cl ratio; (2) saline water with higher Na/Cl (> 0.6), So₄/Cl, and B/Cl ratios. These chemical compositions resemble Ca-chloride saline waters found in other locations in the Coastal Plain aquifer and in underlying formations. The saline water bodies may occur in either pockets at the bottom of the aquifer or lumachelle and sandstone layers of high hydraulic conductivity in underlying sediments.     

Temporary and definitive fixation of atmospheric lead in deep-sea sediments of the Western Mediterranean Sea

Most lead brought to the Mediterranean Sea has an anthropogenic origin and is mainly transported through the atmosphere. Atmospheric Pb was continuously collected at Cap Ferrat in 1986 and 1987. From this study, the estimation of the anthropogenic Pb flux on the whole Western Mediterranean was, averaged on 1986 and 1987 data, 4080 t. Assuming that the atmospheric anthropogenic Pb input varied in this course of time similarly to the consumption of Pb added to gasolines in France, the mean annual flux could be calculated: 3.95 kg km⁻² yr⁻¹, that is an annual input of 3360 t yr⁻¹. Reaching the sea, this metal seems to become rapidly bound to phytoplankton. Grazing by zooplankton leads to the production of faecal pellets which frequently contain rather high metal concentrations. The sinking rate of pellets of various zooplankton species is high; within a few days pellets may reach deep-sea sediments. After deposition, Pb is released from this organic-rich material during early diagenesis. In most cases, it, therefore, returns to the overlaying water body by ascending diffusion. But, in a deep-sea area of approximately 80 000 km² where Mn oxide precipitation occurs in surficial sediments, Pb seems to remain stored by coprecipitation processes. By considering the lead stored in ‘excess’ in the surficial sediment of the deep-sea area, we estimate that a mean annual anthropogenic Pb amount ranging from 800 up to 1080 t was stored every year from 1950. On the same area, taking into account the Pb loss at the straits, the ‘direct’ atmospheric input to the sea bottom is, on average, 184 t yr⁻¹. The remaining part, that is (800–1080)−184=(616–896) t yr⁻¹, corresponds to an additional ‘indirect’ Pb flux in water due to Pb released from sediments of the surrounding areas where it does not remain stored.     

Hydraulic properties of groundwater systems in the saprolite and sediments of the wheatbelt, Western Australia

Hydraulic properties of deeply weathered basement rocks and variably weathered sedimentary materials were measured by pumping and slug-test methods. Results from over 200 bores in 13 catchments, and eight pumping-test sites across the eastern and central wheatbelt of Western Australia were analysed. Measurements were made in each of the major lithological units, and emphasis placed on a ubiquitous basal saprolite aquifer. Comparisons were made between alternative drilling and analytical procedures to determine the most appropriate methods of investigation.

Aquifers with an average hydraulic conductivity of 0.55 m day⁻¹ occur in variably weathered Cainozoic sediments and poorly weathered saprolite grits (0.57 m day⁻¹). These aquifers are separated by an aquitard (0.065 m day⁻¹) comprising the mottled and pallid zones of the deeply weathered profile. Locally higher values of hydraulic conductivity occur in the saprolite aquifer, although after prolonged periods of pumping the values decrease until they are similar to those obtained from the slug-test methods. Hydraulic conductivities measured in bores drilled with rotary auger rigs were approximately an order of magnitude lower than those measured in the same material with bores drilled by the rotary air-blast method.

Wheatbelt aquifers range from predominantly unconfined (Cainozoic sediments), to confined (saprolite grit aquifer). The poorly weathered saprolite grit aquifer has moderate to high transmissivities (4–50 m² day⁻¹) and is capable of producing from less than 5 to over 230 kl day⁻¹ of ground water, which is often of a quality suitable for livestock. Yields are influenced by the variability in the permeability of isovolumetrically weathered materials from which the aquifer is derived.

The overlying aquitard has a low transmissivity (< 1 m² day⁻¹), especially when deeply weathered, indurated and silicified. The transmissivity of the variably weathered sedimentary materials ranges from less than 0.5 m² day⁻¹ to over 10 m² day⁻¹, depending on the texture of the materials and their position within the landscape. Higher transmissivity zones may occur as discrete layers of coarser textured materials. The salinity of the saprolite and sedimentary aquifers ranges from less than 2000 mgl⁻¹ to greater than 250000 mgl⁻¹ (total dissolved solids; TDS), depending on position within the landscape. Secondary soil salinization develops when groundwater discharge occurs from either saprolite or sedimentary aquifers.     

Hydrochemical and stable isotope evidence for the extent and nature of the effective Chalk aquifer of north Norfolk, UK

In eastern England the Chalk aquifer is covered by extensive Pleistocene deposits which influence the hydraulic conditions and hydrochemical nature of the underlying aquifer. In this study, the results of geophysical borehole logging of groundwater temperature and electrical conductivity and depth sampling for major ion concentrations and stable isotope compositions (δ¹⁸O and δ²H) are interpreted to reveal the extent and nature of the effective Chalk aquifer of north Norfolk. It is found that the Chalk aquifer can be divided into an upper region of fresh groundwater, with a Cl concentration of typically less than 100 mg l⁻¹, and a lower region of increasingly saline water. The transition between the two regions is approximately 50 m below sea-level, and results in an effective aquifer thickness of 50–60 m in the west of the area, but less than 25 m where the Eocene London Clay boundary is met in the east of the area. Hydrochemical variations in the effective aquifer are related to different hydraulic conditions developed in the Chalk. Where the Chalk is confined by low-permeability Chalky Boulder Clay, isotopically depleted groundwater (δ¹⁸O less than −7.5‰) is present, in contrast to those areas of unconfined Chalk where glacial deposits are thin or absent (δ¹⁸O about −7.0‰). The isotopically depleted groundwater is evidence for groundwater recharge during the late Pleistocene under conditions when mean surface air temperatures are estimated to have been 4.5°C cooler than at the present day, and suggests long groundwater residence times in the confined aquifer. Elevated molar Mg:Ca ratios of more than 0.2 resulting from progressive rock-water interaction in the confined aquifer also indicate long residence times. A conceptual hydrochemical model for the present situation proposes that isotopically depleted groundwater, occupying areas where confined groundwater dates from the late Pleistocene, is being slowly modified by both diffusion and downward infiltration of modem meteoric water and diffusive mixing from below with an old saline water body.     

Atoll island hydrogeology: flow and freshwater occurrence in a tidally dominated system

A layered-aquifer model of groundwater occurrence in an atoll island was tested with a solute-transport numerical model. The computer model used, SUTRA, incorporates density-dependent flow. This can be significant in freshwater-saltwater interactions associated with the freshwater lens of an atoll island. Boundary conditions for the model included ocean and lagoon tidal variations. The model was calibrated to field data from Enjebi Island, Enewetak Atoll, and tested for sensitivity to a variety of parameters. This resulted in a hydraulic conductivity of 10 m day⁻¹ for the surficial aquifer and 1000 m day⁻¹ for the deeper aquifer; this combination of values gave an excellent reproduction of the tidal response data from test wells. The average salinity distribution was closely reproduced using a dispersivity of 0.02m. The computer simulation quantitatively supports the layered-aquifer model, including under conditions of density-dependent flow, and shows that tidal variations are the predominant driving force for flow beneath the island. The oscillating, vertical flow produced by the tidal variations creates an extensive mixing zone of brackish water. The layered-aquifer model with tidally driven flow is a significant improvement over the Ghyben-Herzberg-Dupuit model as it is conventionally applied to groundwater studies for many Pacific reef islands.     

The physico-chemical conditions of Turkwel Gorge Reservoir, a new man made lake in Northern Kenya

Variations in some physical, chemical, and nutrient conditions were investigated at Turkwel Gorge Reservoir and its inflowing river, Suam between 1994 and 1995. Seasonal changes in inflow volume had the greatest impact on the reservoir and river conditions investigated. A wide fluctuation in inflow volume combined with a regulated outflow independent of season resulted in a draw down of over 10 m in each year. Flood inflows during the wet season resulted in the lowest values of Secchi depth (range, 0.09–2.16 m), electrical conductivity (EC, range = 140–200 mS cm⁻¹) and total alkalinity (TA, range = 75–111 mg l⁻¹) while the highest values were measured during the dry season. A functional relation between EC and TA (TA = 0.529 mg l⁻¹, EC: R² = 0.876) suggests a predominance of carbonates among the anions. Vertical profiles of temperature and dissolved oxygen (DO) revealed that the reservoir is monomictic with a wide variation in the depth of the daily mixed layer. High values of pH (range = 6.7–8.9) and DO (range = 4.9–9.2 mg l⁻¹) were associated with periods of peak phytoplankton photosynthesis while the lowest values followed reservoir mixing. Peak total nitrogen (TN, range = 119–526 μg l⁻¹) and total phosphorus (TP, range = 8.9–71.6 μg l⁻¹) levels during the wet season resulted from increased river loading. Values of dissolved reactive silica (DRS, range = 0.41–9.77 mg l⁻¹) showed a wet season decline which was related to diatom depletion during the wet season. Annual reservoir areal loading rates of 27.38, 10.90 and 408.5 mg m⁻² were computed for TN, TP and DRS respectively based on estimates of inflowing river loads in 1994.

At the inflowing river Suam, low levels of EC (range = 107–210 μS cm⁻¹) and TA (range = 62–125 mg l⁻¹) occurred during the wet season while the highest levels occurred shortly before the river dried up. The first flood water at the resumption of river inflow in March was characterized by very low levels of DO (range = 1.8–8.2 mg l⁻¹) and high levels of TN (range = 205–3354 μg l⁻¹) and TP (102–1259 μg l⁻¹). River pII (6.9–7.7) and DRs (range = 9.01–19.93 mg l⁻¹) varied irregularly throughout the year.     

Estimation of groundwater evaporation and salt flux from Owens Lake, California, USA

Groundwater evaporation and subsequent precipitation of soluble salts at Owens Lake in eastern California have created one of the single largest sources of airborne dust in the USA, yet the evaporation and salt flux have not been fully quantified. In this study, we compare eddy correlation, microlysimeters and solute profiling methods to determine their validity and sensitivity in playa environments. These techniques are often used to estimate evaporative losses, yet have not been critically compared at one field site to judge their relative effectiveness and accuracy. Results suggest that eddy correlation methods are the most widely applicable for the variety of conditions found on large playa lakes. Chloride profiling is shown to be highly sensitive to thermal and density-driven fluxes in the near surface and, as a result, appears to underestimate yearly groundwater evaporation. Yearly mean groundwater evaporation from the playa surface estimated from the three study areas was found to range from 88 to 104 mm year⁻¹, whereas mean evaporation from the brine-covered areas was 872 mm year⁻¹. Uncertainties on these mean rates were estimated to be ±25%, based on comparisons between eddy correlation and lysimeter estimates. On a yearly basis, evaporation accounts for approximately 47 × 10⁶ m³ of water loss from the playa surface and open-water areas of the lake. Over the playa area, as much as 7.5 × 10⁸ kg (7.5 × 10⁵ t) of salt are annually concentrated by evaporation at or near the playa surface, much of which appears to be lost during dust storms in area.     

Tracing groundwater flow in the Borden aquifer using krypton-85

Krypton-85 was measured in air, soil gas, and ground water at the Borden aquifer in Ontario in October 1989. The measured specific activities in air and soil gas were 52.0 ± 2.0 and 53.6 ± 1.8 disintegrations per min (dpm) cm⁻³ krypton. These measurements are in excellent agreement with the global atmospheric trend and demonstrate that krypton-85 enters the water table at the Borden site without a lag in the soil gas reservoir. The krypton-85 specific activity in five groundwater samples ranged from 44.9 to 9.5 dpm cm⁻³ corresponding to groundwater ages of 2–17 years with a monotonic decrease in specific activity (increase in age) along the groundwater flow path. Travel times calculated from a two-dimensional steady-state model of groundwater flow agree well with the krypton-85 ages in the main recharge region of the aquifer where flow is predominantly vertical, but were 30–40% older than the krypton-85 age downstream of the main recharge area where the flow is mainly horizontal. The effect of dispersion on the distribution of krypton-85 was determined by modelling the transport of krypton-85 in the Borden aquifer with a two-dimensional time-dependent advection dispersion model using the steady-state flow field. Agreement between model specific activity and observed specific activity was excellent for samples in the main recharge region, but the model specific activities were 30–50% lower than observed specific activities in the region of horizontal flow. The differences in travel times and krypton-85 ages and in model krypton-85 and observed krypton-85 specific activities are considered to be small given the heterogeneities that exist in the hydraulic conductivity and aquifer geometry and hence in the groundwater flow field. The model simulated krypton-85 distribution was not sensitive to changes in longitudinal dispersivity and was only weakly sensitive to changes in transverse dispersivity. The geochemical inertness, well-defined source function, and insensitivity to dispersion of krypton-85 allow estimates of groundwater age to be made in a straightforward manner and measurement of krypton-85 can significantly enhance the characterization of groundwater flow in many shallow subsurface systems.     

Ecological–economic modeling as a tool for watershed management: A case study of Lake Qionghai watershed, China

This paper presents an ecological–economic model for a lake and its watershed systems. We describe the linkage between the watershed system and the lake aquatic ecosystem and the modeling process. The lake–watershed system was divided into six subsystems: social system, economic system, terrestrial ecosystem, lake water system, pollutant system, and lake aquatic ecosystem. The model equations were constructed based on five main assumptions. The Lake Qionghai watershed in southwestern China, which is undergoing rapid eutrophication, was used as a case study. The targeted goals for total phosphorus (TP) and chlorophyll a (Chl a) concentrations in the lake in 2015 are 0.025 and 10.0 mg m⁻³, respectively. We present two scenarios from 2004 to 2015 based on the ecological–economic model. In both scenarios, the TP and Chl a concentrations in the lake are predicted to increase under the effects of watershed pressures and the targeted goals cannot be met. The application of techniques to reduce pollutants loading and the corresponding pollutants reductions are reflected again in the constructed model. The model predicts that TP and Chl a concentrations will decrease to 0.024 and 7.71 mg m⁻³, respectively, which meet the targeted thresholds. The model results provide directions for local government management of watersheds and lake aquatic ecosystem restoration.     

Groundwater-supported evapotranspiration within glaciated watersheds under conditions of climate change

This paper analyzes the effects of geology and geomorphology on surface-water/-groundwater interactions, evapotranspiration, and recharge under conditions of long-term climatic change. Our analysis uses hydrologic data from the glaciated Crow Wing watershed in central Minnesota, USA, combined with a hydrologic model of transient coupled unsaturated/saturated flow (HYDRAT2D). Analysis of historical water-table (1970–1993) and lake-level (1924–2002) records indicates that larger amplitude and longer period fluctuations occur within the upland portions of watersheds due to the response of the aquifer system to relatively short-term climatic fluctuations. Under drought conditions, lake and water-table levels fell by as much as 2–4 m in the uplands but by 1 m in the lowlands. The same pattern can be seen on millennial time scales. Analysis of Holocene lake-core records indicates that Moody Lake, located near the outlet of the Crow Wing watershed, fell by as much as 4 m between about 4400 and 7000 yr BP. During the same time, water levels in Lake Mina, located near the upland watershed divide, fell by about 15 m. Reconstructed Holocene climate as represented by HYDRAT2D gives somewhat larger drops (6 and 24 m for Moody Lake and Lake Mina, respectively). The discrepancy is probably due to the effect of three-dimensional flow. A sensitivity analysis was also carried out to study how aquifer hydraulic conductivity and land-surface topography can influence water-table fluctuations, wetlands formation, and evapotranspiration. The models were run by recycling a wet year (1985, 87 cm annual precipitation) over a 10-year period followed by 20 years of drier and warmer climate (1976, 38 cm precipitation). Model results indicated that groundwater-supported evapotranspiration accounted for as much as 12% (10 cm) of evapotranspiration. The aquifers of highest hydraulic conductivity had the least amount of groundwater-supported evapotranspiration owing to a deep water table. Recharge was even more sensitive to aquifer hydraulic conductivity, especially in the lowland regions. These findings have important implications for paleoclimatic studies, because the hydrologic response of a surface-water body will vary across the watershed to a given climate signal.     

Comparison of 15 evaporation methods applied to a small mountain lake in the northeastern USA

Few detailed evaporation studies exist for small lakes or reservoirs in mountainous settings. A detailed evaporation study was conducted at Mirror Lake, a 0.15 km² lake in New Hampshire, northeastern USA, as part of a long-term investigation of lake hydrology. Evaporation was determined using 14 alternate evaporation methods during six open-water seasons and compared with values from the Bowen-ratio energy-budget (BREB) method, considered the standard. Values from the Priestley–Taylor, deBruin–Keijman, and Penman methods compared most favorably with BREB-determined values. Differences from BREB values averaged 0.19, 0.27, and 0.20 mm d⁻¹, respectively, and results were within 20% of BREB values during more than 90% of the 37 monthly comparison periods. All three methods require measurement of net radiation, air temperature, change in heat stored in the lake, and vapor pressure, making them relatively data intensive. Several of the methods had substantial bias when compared with BREB values and were subsequently modified to eliminate bias. Methods that rely only on measurement of air temperature, or air temperature and solar radiation, were relatively cost-effective options for measuring evaporation at this small New England lake, outperforming some methods that require measurement of a greater number of variables. It is likely that the atmosphere above Mirror Lake was affected by occasional formation of separation eddies on the lee side of nearby high terrain, although those influences do not appear to be significant to measured evaporation from the lake when averaged over monthly periods.     

Before and after riparian management: sediment and nutrient exports from a small agricultural catchment, Western Australia

Riparian vegetation can trap sediment and nutrients sourced from hillslopes and reduce stream bank erosion. This study presents results from a 10-year stream monitoring program (1991–2000), in a 6 km² agricultural catchment near Albany, Western Australia. After 6 years, a 1.7 km stream reach was fenced, planted with eucalyptus species and managed independently from the adjacent paddocks. Streamflow, nutrient and sediment concentration data were collected at the downstream end of the fenced riparian area, so there are data for before and after improved riparian management. Suspended sediment (SS) concentrations fell dramatically following improved riparian management; the median event mean concentration (EMC) dropped from 147 to 9.9 mg l⁻¹. Maximum SS concentrations dropped by an order of magnitude. As a result, sediment exports from the catchment decreased following improved riparian management, from over 100 to less than 10 kg ha⁻¹ yr⁻¹. Observations suggest that this was the result of reduced bank erosion and increased channel stability. Riparian management had limited impact on total phosphorus (TP) concentrations or loads, but contributed to a change in phosphorus (P) form. Before improved riparian management, around half of the P was transported attached to sediment, but after, the median filterable reactive P (FRP) to TP ratio increased to 0.75. In addition, the median FRP EMC increased by 60% and the raw median FRP concentration increased from 0.18 to 0.35 mg l⁻¹. These results suggest that there was a change in the dominant P form, from TP to FRP. Changes in total nitrogen (TN) following improved riparian management were less clear. There were reductions in TN concentrations at high flows, but little change in the loads or EMC. This study demonstrates the benefits of riparian management in reducing stream bank erosion, but suggests that in catchments with sandy, low P sorption soils, there may be limitations on the effectiveness of riparian buffers for reducing P and N exports.     

Use of statistical analysis to formulate conceptual models of geochemical behavior: water chemical data from the Botucatu aquifer in São Paulo state, Brazil

Water chemical data from the Botucatu Sandstone aquifer in the São Paulo State part of the Paraná Basin, Brazil, was evaluated using geochemical methods and two statistical analyses: cluster analysis and factor analysis. The results were used to develop a conceptual geochemical model, in which three geochemical regions were identified, and their chemical behavior was modeled. The characteristic chemicals, changing from the recharge area to the center of the basin, are: SiO₂—(HCO₃⁻ and Ca²⁺)—(Na⁺, CO₃²⁻, and SO₄²⁻). The distribution of the chemicals is interpreted as controlled by different water–rock interaction processes in the different regions. In the recharge area, dissolution of alkali–feldspar minerals in the sandstone is the main reaction observed; in the mid-section of the basin, calcite dissolution results in high calcium and bicarbonate concentration; in the center of the basin, leakage from underlying layers becomes the governing factor.