Subsurface Discharge Research Articles

Laboratory and numerical investigations of hillslope soil saturation development and runoff generation over rainfall events

Summary Runoff is a key process that controls the behaviour of a hillslope hydrological system. The study presented here aims to examine the mechanisms of runoff generation due to saturation excess by investigating the development of the subsurface saturated zone. Experiments were conducted on a hillslope system with a uniform slope (0.2) and a relatively homogeneous and highly permeable medium ( K s = 1.28 cm/min), subjected to high and constant surface recharges (0.51–0.69 cm/min). Two initial conditions prior to the rainfall events were set up in the experiments to represent relatively dry and wet antecedent soil conditions, respectively. Measurements showed that during vertical infiltration, local pressure head and soil moisture remained constant for a certain period, showing a ‘waiting’ behaviour. The saturated area formed initially at the slope toe, quickly rose to the surface and subsequently expanded to the upslope. When propagating in the upslope direction, the wetting front caused the pore-water flow to deflect in areas above the wetting front and at the slope base. With a wetter initial condition, the soil responded to the rainfall more quickly. The initial moisture conditions also altered the relation between the subsurface discharge and pressure head. Under the applied rainfall rates, the system reached a fully saturated condition and produced surface runoff. The rainfall intensity was found to affect the temporal variations and magnitude of surface runoff characteristics; however it did not seem to impose any significant effect on the maximum subsurface discharge rate. These results provide insight into the behaviour of the hillslope system in response to rainfall.

Permafrost degradation and subsurface-flow changes caused by surface warming trends

Change dynamics of permafrost thaw, and associated changes in subsurface flow and seepage into surface water, are analysed for different warming trends in soil temperature at the ground surface with a three-phase two-component flow system coupled to heat transport. Changes in annual, seasonal and extreme flows are analysed for three warming-temperature trends, representing simplified climate-change scenarios. The results support previous studies of reduced temporal variability of groundwater flow across all investigated trends. Decreased intra-annual flow variability may thus serve as an early indicator of permafrost degradation before longer-term changes in mean flows are notable. This is advantageous since hydrological data are considerably easier to obtain, may be available in longer time series, and generally reflect larger-scale conditions than direct permafrost observations. The results further show that permafrost degradation first leads to increasing water discharge, which then decreases as the permafrost degradation progresses further to total thaw. The most pronounced changes occur for minimum annual flows. The configuration considered represents subsurface discharge from a generic heterogeneous soil-type domain.

Constraining saturated hydraulic transmissivity values by means of maximum saturated area mapping and maximum subsurface discharge: long‐term simulation in the Amazon basin

AbstractAlthough saturated hydraulic conductivity (K0) and saturated hydraulic transmissivity (T0) are physically based parameters, quite often, their calibrated values lie out of the physically based range, driving modellers to increased K0 values in some orders of magnitude, from field measured values, in order to fit observed discharge data. Therefore, calibration is a necessary process in hydrological modelling. Within a hydrological model, K0 or T0 plays an important role. They allow the model to represent the hydrograph separation through estimates of subsurface discharges. Simulating different flow components in hydrological modelling is an increasing need for basin management and water quality predictions. The present study has the objective to develop two criteria to constrain T0 values using a TOPMODEL multi‐velocity approach in order to represent the hydrograph separation in a more realistic way. The method was (i) applied to three sub‐basins in the Amazon basin (the Óbidos, the Humaita and Santo Antônio basins), (ii) evaluated by means of a combined objective function and uncertainty bounds and (iii) validated in a long‐term simulation. Because the constraint criteria were applied, it has verified a significant reduction in subsurface uncertainty for all the basins. The constraint criteria reduced the number of simulations to 11%, 42% and 23% for the Óbidos basin, the Humaita basin and the Santo Antônio basin, respectively. This means that a large number of simulations were rejected in the sense they represented unrealistic hydrograph separation. Copyright © 2012 John Wiley & Sons, Ltd.

Submarine underground discharge and possible mechanism responsible for dissociation of marine methane hydrates in the Arctic region

Groundwater discharge represents the least studied element of the hydrological cycle, the dynamics of which is responsible in some areas for generating regionally and globally significant different types of climatic feedback [1, 2]. The groundwater influx to seas and oceans is realized through three principally different ways: in the form of juvenile waters, with river flow, and through submarine discharge of groundwater that is formed in terrestrial conditions and discharges immediately in seas avoiding the river network. According to available estimates, the global influx of juvenile waters to the Earth’s surface is up to 1k m 3 /year [3]. The groundwater influx to seas and oceans with the river runoff is realized through groundwater draining by rivers. This component of the subsurface discharge is a constituent of the total river runoff and is rather correctly taken into account in balance calculations. The submarine groundwater dis� charge in the zone of intense and slow water exchange is the most complex and insufficiently studied process. The groundwater recharge and discharge intensities in this zone are determined by climatic variations on the decadal and centennial scales. The climatic feedback reflecting the influence of the groundwater runoff on dynamics of the climatic system has never been con� sidered. The regional assessment of the submarine dis� charge is based on the quantitative analysis of ground�

Modelling the impact of a subsurface barrier on groundwater flow in the lower Palar River basin, southern India

Groundwater modelling is widely used as a management tool to understand the behaviour of aquifer systems under different hydrological stresses, whether induced naturally or by humans. The objective of this study was to assess the effect of a subsurface barrier on groundwater flow in the Palar River basin, Tamil Nadu, southern India. Groundwater is supplied to a nearby nuclear power plant and groundwater also supplies irrigation, industrial and domestic needs. In order to meet the increasing demand for groundwater for the nuclear power station, a subsurface barrier/dam was proposed across Palar River to increase the groundwater heads and to minimise the subsurface discharge of groundwater into the sea. The groundwater model used in this study predicted that groundwater levels would increase by about 0.1–0.3 m extending out a distance of about 1.5–2 km from the upstream side of the barrier, while on the downstream side, the groundwater head would lower by about 0.1–0.2 m. The model also predicted that with the subsurface barrier in place the additional groundwater requirement of approximately 13,600 m3/day (3 million gallons (UK)/day) can be met with minimum decline in regional groundwater head.

Modeling heat fluxes in macroporous soil under sparse young forest of temperate humid climate

This research is focused on improved quantitative understanding of processes controlling the movement of water and heat below the land surface under natural vegetation. Special attention is paid to the advective transport of soil heat during rainfall events and the role of preferential flow in the soil heat transfer system. Transient flow of water and transport of heat were simulated using dual-continuum model with semi-separate flow domains representing the soil matrix and the network of preferential pathways. The model was able to reproduce quite well the measured variations of soil temperatures, and, at least in principle, also the measured temperatures of shallow subsurface discharge, closely associated with the preferential flow.

Optimization of pumping rate and recharge through numerical modeling with special reference to small coral island aquifer

The groundwater is the only source of availability of fresh water in tiny coral islands. In the past decades, there has been growing demand for fresh water to meet the need of domestic besides other purposes. The aquifer system on these islands is fragile besides being subjected to various stresses like high subsurface discharge, increased abstraction, improper disposal of waste water and tidal waves of ocean all of which subject the aquifer prone to sea water intrusion and thus reduction and deterioration the water quality. Therefore, understanding the aquifer’s behavior and then work out a sustainable option for fresh water is essential. The paper concerns optimizing of pumping and artificial recharge paces to reduce the effects of various stresses over tiny and fragile lens-shaped coral island aquifer system. The density driven ground water flow was simulated using SEAWAT (MODFLOW and MT3D based computer program) model. Detailed hydrogeological investigations were carried out to determine the quantity of freshwater that could be pumped to avoid the seawater intrusion into the aquifer through modeling. Initial heads, physical parameters and boundary conditions of the study area have been defined in the model based on field data, geophysical measurements and interpretations and hydrogeological studies. The model was calibrated by obtaining a match of computed and observed values of the water table, as hydraulic head is much more sensitive to pumping rates than any other stress. A few sentences about: flow model were utilized to derive optimal pumping rate; the effect of artificial recharge through the model, has also proved that the salt-water intrusion could be stopped by raising the water level through temporarily storing the artificially recharged water post construction of subsurface dam near the coast.

Submarine discharge into the seas of the Arctic Ocean from the European Russia territory

Groundwater formation conditions in the upper hydrodynamic zone of the northern coast of European Russia are considered. This groundwater discharges directly into the Barents and White seas. The values of submarine discharge from European Russia into arctic seas, bypassing river network, are estimated. Estimates of subsurface dissolved-solids discharge are given. Specific and integral characteristics of submarine discharge are analyzed. The major regularities in the formation and distribution of submarine discharge into seas of the Arctic Ocean are described.

SUBSURFACE CHEMICAL DISCHARGE IN LITHUANIAN AREA

An assessment of groundwater chemical leakage under the Lithuanian hydrogeological conditions, using the classic method of hydrogeological/ hydrograph division and linking it with the chemical leakage. Subsurface chemical discharge is the amount of salts flown by the rivers due to their draining impact on the subsurface. The chemical runoff is determined by two key factors: groundwater runoff yield and total content of dissolved solids (TDS). The value of the groundwater runoff module (the yield from 1 km2) in the river basins of Lithuania ranges from 0.4 to 5.0 l/s km2. TDS values in shallow groundwater drained by the rivers range from 180 to 800 mg/l. The modules of subsurface chemical runoff in the area of Lithuania range from 3–9 to 54 t/year from 1 km2 with the highest values observed in the Baltija uplands and Dainava Plain. During the last decade, the chemical runoff has stabilised due to decline in technogenic load. If compared to the dissolved solids drained by the rivers the subsurface chemical discharge can make up 7–45%. The chemical runoff out for all area of Lithuania, as assessed by the hydrological/hydrogeological technique according to the minimum long‐term runoff and TDS content in the river water of that period, reaches 2.2 mln. t/year of mineral material, about 90% of which come from the Nemunas River basin. To restrict the influence of technogenic pollution on the results of the assessment of underground chemical runoff, only observations done upstream the pollution sources (mainly urban) have been used. The qualitative assessment of the changes in groundwater chemical discharge and flow fluctuations due to technogenic impact has been carried out by applying graphical analysis of the underground chemical runoff module. Santrauka Darbo tikslas – ivertinti požemini chemini nuoteki Lietuvos hidrogeologinemis salygomis, taikant klasikine hidrologohidrogeologine hidrografo skaidymo metodika. Požeminis cheminis nuotekis – tai drusku kiekis, nutekantis upemis del drenažinio poveikio. Chemini nuoteki lemia du pagrindiniai veiksniai – požeminio nuotekio debitas ir vandens bendroji mineralizacija. Požeminio nuotekio modulio dydis (debitas iš 1 km2) Lietuvos teritorijos upiu baseinuose kinta nuo 0,4 iki 5,0 l/s km2. I upes požemiu nutekančio gruntinio vandens bendroji mineralizacija – nuo 180 iki 800 mg/l. Požeminio cheminio nuotekio Lietuvos teritorijoje moduliai yra nuo 3–9 iki 54 t/metus iš 1 km2. Didžiausias požeminis cheminis nuotekis Baltijos aukštumose bei Dainavos lygumoje. Pastaraji dešimtmeti cheminis nuotekis stabilizavosi del sumažejusios technogenines apkrovos. Palyginti su bendru mineraliniu medžiagu kiekiu, nutekančiu upemis, požeminis cheminis nuotekis gali sudaryti 7–45 %. Visas požeminis cheminis nuotekis Lietuvos teritorijoje, ivertintas pagal minimalu daugiameti nuoteki ir to periodo upiu vandens mineralizacija, siekia 2,2 mln. t/metus mineraliniu medžiagu, kuriu apie 90 % išplukdoma iš Nemuno baseino. Technogeninio užterštumo itakai požeminio cheminio nuotekio vertinimo rezultatams riboti naudoti tik stebejimu duomenys aukščiau taršos šaltiniu (daugiausia miestu). Požeminio vandens cheminio nuotekio, kuri lemia technogeninis poveikis, pokyčiu kokybinis ivertinimas atliktas nuotekio moduliu grafoanalizes pagrindu. Резюме Целью работы было оценить подземный химический сток с использованием расчленения гидрографа рек и выноса химического материала в гидрогеологических условиях Литвы. Подземный химический сток – это количество солей, протекающих в речных водах из-за их дренажного воздействия. Химический сток зависит от двух основных факторов: дебита подземного стока и общей минерализации воды. Модуль подземного стока (дебит с 1 км2 ) речных бассейнов в пределах Литвы меняется от 0,4 до 5,0 л/с с 1 км2 . Общая минерализация грунтовых вод, притекающих в реки, составляет 180–800 мг/л. Модули подземного химического стока в пределах территории Литвы имеют значения от 3–9 до 54 т/год с 1 км2 . Наибольший подземный химический сток наблюдается на терригории Балтийских высот и Дайнавской равнины. В последнее десятилетие химический сток стал более стабильным из-за уменьшающейся техногенной нагрузки. По сравнению с общим стоком минеральных веществ в реках подземный химический сток может составляет 7–45%. Весь подземный химический сток, оцененный по минимальному многолетнему стоку и минерализации речной воды, составляет 2,2 млн. т/год минерального вещества, из которого 90% поступает из бассейна р. Нямунас. Для уменьшения техногенного воздействия на результаты расчетов использованы данные химического состава воды только с постов наблюдения, расположенных выше источников загрязнения. Оценка изменения подземного химического стока выполнена графоаналитически с использованием изменения его модуля во времени.

Hydrological Effects of Bounding the Venice (Italy) Industrial Harbor by a Protection Cutoff Wall: Modeling Study

To prevent erosion of polluted land and discharge of contaminated groundwater and surface water into the Venice Lagoon, a 57 km long cutoff wall has been designed by the Venice Water Authority and is currently under construction along the canal banks of the Venice, Italy industrial harbor. To predict the impact on the hydrologic regime and mitigate the related inundation hazard in the nearby urban areas, a complex three-dimensional finite-element model has been developed and implemented over the multiaquifer system down to a depth significantly larger than the wall bottom. The model is initially calibrated against the regional piezometry and then refined locally to reproduce the groundwater volume drained along a 5-km long bank of a harbor canal already bounded. Major results from the simulations show that after the wall completion the subsurface discharge into the lagoon is successfully abated by as much as 85% relative to the preexisting rate with, however, an expected 1-m increase of the water table in the inland city of Mestre, Italy thus pointing to the need for implementing a drainage trench upstream the wall to properly reduce the raised groundwater level. The model would also indicate that the shallowest aquifers may receive underground water from a large number of old deeper boreholes abandoned in the past and not properly sealed. From a more general perspective, the present study constitutes an important example where the reliable and sustainable design of a complex engineered structure bound to impact significantly on the surrounding environment can be much helped and improved by the use of advanced numerical models capable to capture the essential features of the underlying geohydrological processes.

Comparative predictions of discharge from an artificial catchment (Chicken Creek) using sparse data

Abstract. Ten conceptually different models in predicting discharge from the artificial Chicken Creek catchment in North-East Germany were used for this study. Soil texture and topography data were given to the modellers, but discharge data was withheld. We compare the predictions with the measurements from the 6 ha catchment and discuss the conceptualization and parameterization of the models. The predictions vary in a wide range, e.g. with the predicted actual evapotranspiration ranging from 88 to 579 mm/y and the discharge from 19 to 346 mm/y. The predicted components of the hydrological cycle deviated systematically from the observations, which were not known to the modellers. Discharge was mainly predicted as subsurface discharge with little direct runoff. In reality, surface runoff was a major flow component despite the fairly coarse soil texture. The actual evapotranspiration (AET) and the ratio between actual and potential ET was systematically overestimated by nine of the ten models. None of the model simulations came even close to the observed water balance for the entire 3-year study period. The comparison indicates that the personal judgement of the modellers was a major source of the differences between the model results. The most important parameters to be presumed were the soil parameters and the initial soil-water content while plant parameterization had, in this particular case of sparse vegetation, only a minor influence on the results.

Coupled modelling of subsurface water flux for an integrated flood risk management

Abstract. Flood events cause significant damage not only on the surface but also underground. Infiltration of surface water into soil, flooding through the urban sewer system and, in consequence, rising groundwater are the main causes of subsurface damage. The modelling of flooding events is an important part of flood risk assessment. The processes of subsurface discharge of infiltrated water necessitate coupled modelling tools of both, surface and subsurface water fluxes. Therefore, codes for surface flooding, for discharge in the sewerage system and for groundwater flow were coupled with each other. A coupling software was used to amalgamate the individual programs in terms of mapping between the different model geometries, time synchronization and data exchange. The coupling of the models was realized on two scales in the Saxon capital of Dresden (Germany). As a result of the coupled modelling it could be shown that surface flooding dominates processes of any flood event. Compared to flood simulations without coupled modelling no substantial changes of the surface inundation area could be determined. Regarding sewerage, the comparison between the influx of groundwater into sewerage and the loading due to infiltration by flood water showed infiltration of surface flood water to be the main reason for sewerage overloading. Concurrent rainfalls can intensify the problem. The infiltration of the sewerage system by rising groundwater contributes only marginally to the loading of the sewerage and the distribution of water by sewerage has only local impacts on groundwater rise. However, the localization of risk areas due to rising groundwater requires the consideration of all components of the subsurface water fluxes. The coupled modelling has shown that high groundwater levels are the result of a multi-causal process that occurs before and during the flood event.

Evaluating current drainage practices and feasibility of controlled drainage in the YinNan Irrigation District, China

The YinNan Irrigation District (YNID) is located in the upper reaches of the Yellow River in NingXia, China. Its drainage system was constructed between the 1950s and 1970s to maintain salt and water balances in the district. The system, however, has been reported draining the agricultural lands excessively, resulting in frequent irrigations and large amount of drainage discharge into the Yellow River each year. This paper presents a comprehensive analysis of current drainage practices in the YNID and discusses the feasibility of controlled drainage in order to mitigate the negative impact of agricultural drainage. Based on historical records and available information, drainage discharge has been estimated for different crop fields and ditches. The results show that for the major water use crop, rice, controlled drainage may reduce subsurface discharge through field ditches up to 94%. For wheat and corn, the benefit of controlled drainage is not significant because the major drainage discharge is directly through the main ditch system and the Yellow River channel. The rotational cropping nature and local water management tradition, however, make controlled drainage a very attractive option for water savings and reducing water pollution to the Yellow River.

Stable water isotopes in the atmosphere/biosphere/lithosphere interface: Scaling-up from the local to continental scale, under humid and dry conditions

At and near the land surface, the precipitation is partitioned into fluxes of percolation, surface runoff and evapo-transpiration. These processes are accompanied by changes in the stable isotopic composition, primarily the result of isotopic fractionation between the water molecules associated with the partial evaporation from open surface and soilwaters. Following such an evaporative process the residual water is marked by an enrichment of the heavy isotopes and decrease of the d-excess parameter, with the complementary opposite effect on the atmospheric moisture. Transpiration, on the other hand, returns the water essentially unfractionated to the atmosphere. This results in a change of the amount of water which is unaccompanied by a change in the isotopic composition and which has been widely used to distinguish between the transpiration and evaporation fluxes. An understanding of the isotopic composition also requires consideration of a possible selection between precipitation events of different isotopic composition on a short term and seasonal basis. With increasing geographic scale, any local effect of the evaporative isotopic signature can be attenuated in the runoff under an exorheic regime by subsequent rainout and the accumulation of additional surface or sub-surface discharges that have not suffered an evaporative water loss; however, since rainout usually occurs under close to equilibrium conditions, the integrated isotope signature of all evaporation processes is preserved in the atmospheric moisture. In the more arid environment with its endorheic runoff regime, more and more of the surface water is lost by evaporation as the scale of the basin increases, often terminating in highly saline lakes. In contrast to the situation described above, the signature of the d-excess is thus accentuated in the runoff whereas its contribution to the atmospheric moisture diminishes as a larger and larger fraction of the surface water is being evaporated.

Quantifying Sub-Surface Discharges from Individual Sewer Defects

Quantifying Sub-Surface Discharges from Individual Sewer Defects

The First Results of an Investigation into the Phylogenetic Diversity of Microorganisms in Southern Baikal Sediments in the Region of Subsurface Discharge of Methane Hydrates

Phylogenetic analysis of the bacterial communities in Lake Baikal bottom sediments in the region of subsurface methane hydrate discharge has been carried out using data on 16S rRNA sequences. The composition of these microbial communities is shown to be different in different horizons. Methanotrophic bacteria are found in the surface layer (0–5 cm), and uncultured bacteria constitute a great portion of this population. In deeper sediment layers (92–96 cm), a change in the microbial community occurs; specifically, a decreased homology with the known sequences is observed. The new sequences form separate clusters on a phylogenetic tree, indicating the possibly endemic nature of the bacteria revealed. Organisms related to the genus Pseudomonas constitute the main portion of the population. An archaea-related sequence was found in a horizon containing gas hydrate crystals (100–128 cm). Uncultured bacteria remain predominant.

Magma intrusion and discharge process at the initial stage of the 2000 activity of Miyakejima, Central Japan, inferred from tilt and GPS data

SUMMARY The 2000 eruptive activity of Miyakejima island began with an earthquake swarm and crustal deformation that was clearly observed by continuous observations of ground tilt and GPS on Miyakejima. Based on the crustal deformation data, we estimated the magma migration process at the initial stage (18:30 LT on 2000 June 26—06:00 LT on 2000 June 27) of the activity. The activity in 2000 has been characterized by a collapse-caldera formation and no fissure eruption on the flank of the island, which is markedly different from the style of the recent eruptions in 1940, 1962 and 1983. We constructed a source model that approximately explains the crustal deformation data. The model consists of four dykes, of which three are intruding dykes and the other is a contracting dyke. We can infer the magma migration process at the initial stage from the model as follows. At 18:30 LT on June 26, the magma began to ascend from a dyke-shaped magma chamber and intrude at the southwestern flank of Miyakejima. In the intrusion, the total volume of intruded magma was ∼4 × 10 6 m 3 .A taround 21:00 LT, a relatively large volume of magma began to intrude as a dyke beneath the west coast of Miyakejima. The dyke then propagated laterally towards the northwest. The amount of magma intruded from 21:00 LT to 01:00 LT was ∼40 × 10 6 m 3 . This large intrusion caused a discharge of magma from the magma chamber. The northwestward propagation of the dyke and the contraction of the chamber continued thereafter. The discharge of magma from the chamber beneath Miyakejima probably starved the first intrusion that had been ascending towards the southwestern flank of the island, resulting in the collapse-caldera formation after the initial stage. The style of the 2000 eruptive activity relative to recent activity has primarily been changed by the subsurface discharge of magma towards the northwest.

Investigation into salinisation in southwestern Queensland

This study, combining geophysical and environmental approaches, was undertaken to investigate the causes of secondary salinity in the Goondoola Basin, in southwestern Queensland. A range of airborne and ground geophysical datasets were acquired, along with information on surface and subsurface materials and groundwater. Relationships established between various spatial and point datasets allowed us to generate predictive maps of surface materials, surficial salt stores and recharge potential. Investigation of groundwater and subsurface architecture identified subsurface salt stores and potential discharge areas. Salinisation in this area occurs as a result of local- and intermediate-scale processes, controlled strongly by regolith architecture. Management strategies can now be tailored specifically to prevent excess recharge and further salt mobilisation.

Effect of plastic-film mulching on leaching of nitrate nitrogen in an upland field converted from paddy

A lysimeter experiment was conducted to examine the effects of plastic film mulching on the leaching rate of nitrate nitrogen (NO3-N) from chemical fertilizer that was applied to an upland field that had been converted from paddy rice production. Leaching was monitored in two lysimeters filled with sandy loam soil, which contained low soil organic matter content, under different surface mulch conditions. One was mulched only on the ridge (ridge-mulch treatment) and another one was mulched fully, including the furrow, with black plastic film (full-mulch treatment). Chemical fertilizer was mixed into the top 0.2 m of soil in the two lysimeters before installing the mulch. After transplanting broccoli, the amount of subsurface discharge water and the NO3-N concentrations in the discharge water were measured every day. Larger NO3-N discharges occurred in the ridge-mulch treatment for three days after heavy rainfalls in which cumulative precipitation exceeded 10 mm, and the daily NO3-N load was twice as large as the full-mulch treatment. The differences in the amount of subsurface discharge water and NO3-N discharged between treatments were not significant when there was no rainfall. Cumulative NO3-N loads for the ridge- and full-mulch treatment during the last month of the experimental period were 0.246 and 0.195 g m−2, respectively. The effect of mulching on the reduction of NO3-N discharge rate was higher for the full-mulch treatment. This result showed that a plastic-film mulching system would be effective as an appropriate fertilizer management to reduce nitrate-leaching losses.

Ground water discharge and nitrate flux to the Gulf of Mexico.

Ground water samples (37 to 186 m depth) from Baldwin County, Alabama, are used to define the hydrogeology of Gulf coastal aquifers and calculate the subsurface discharge of nutrients to the Gulf of Mexico. The ground water flow and nitrate flux have been determined by linking ground water concentrations to 3H/3He and 4He age dates. The middle aquifer (A2) is an active flow system characterized by postnuclear tritium levels, moderate vertical velocities, and high nitrate concentrations. Ground water discharge could be an unaccounted source for nutrients in the coastal oceans. The aquifers annually discharge 1.1 +/- 0.01 x 10(8) moles of nitrate to the Gulf of Mexico, or 50% and 0.8% of the annual contributions from the Mobile-Alabama River System and the Mississippi River System, respectively. In southern Baldwin County, south of Loxley, increasing reliance on ground water in the deeper A3 aquifer requires accurate estimates of safe ground water withdrawal. This aquifer, partially confined by Pliocene clay above and Pensacola Clay below, is tritium dead and contains elevated 4He concentrations with no nitrate and estimated ground water ages from 100 to 7000 years. The isotopic composition and concentration of natural gas diffusing from the Pensacola Clay into the A3 aquifer aids in defining the deep ground water discharge. The highest 4He and CH4 concentrations are found only in the deepest sample (Gulf State Park), indicating that ground water flow into the Gulf of Mexico suppresses the natural gas plume. Using the shape of the CH4-He plume and the accumulation of 4He rate (2.2 +/- 0.8 microcc/kg/1000 years), we estimate the natural submarine discharge and the replenishment rate for the A3 aquifer.