Hydrological analysis as a technical tool to support strategic and economic development: A case study of Lake Navaisha,Kenya

Effective integrated water resources management requires reliable estimation of an overall basin water budget and of hydrologic fluctuations between groundwater and surface‐water resources. Seasonal variability of groundwater‐surface water exchange fluxes impacts on the water balance. The long term lake water balance was calculated by Modflow using the stage‐volume rating curve of Lake Package LAK3. The long term average storage volume change is 8.4 × 10⁸ m³/month. The lake water balances suggests that the lake is not in equilibrium with the inflow and outflow terms. Using field abstraction data analysis and model simulation, the combined volume of lake‐groundwater used for industrial abstraction since the last three decades was estimated. This requires an average abstraction amount of 7.0 × 10⁶ m³/month with a long term trend of abstraction ratio 30% (groundwater) and 70% (lake water) since 1980. The amount resulted in a lake which might have been 4.8 m higher than was observed in the last stress period (2010). A long term regional groundwater budget is calculated reflecting all water flow in to and out of the regional aquifer. The model water balance suggests that lake Navaisha basin is in equilibrium with a net outflow about 1% greater than the inflow over the calibrated period of time (1932–2010). The regional model is best used for broad‐scale predictions and can be used to provide a general sense of groundwater to surface water and groundwater to groundwater impacts in the basin. A basin wide water resource management strategy can be designed by integrating the lake/wetland within the regional groundwater model to increase the level of sustainable production and good stewardship in Lake Navaisha. Such hydrological analysis is crucial in making the model serve as simulator of the response of lake stage to hydraulic stresses applied to the aquifer and variation in climatic condition.     

Ermittlung der hydraulischen Durchlässigkeit von quartären Sedimenten mittels Siebanalysen und Pumpversuchen

Fluvial sediments in the Babenhausen area (southeast of Frankfurt am Main) form an important reservoir of sand and gravel, which is currently being mined in dredged lakes. As the fluvial sediments also contain a productive aquifer which is intensely used, possible contamination of the groundwater by sand and gravel mining must be prevented. In order to estimate risk potential, the spatial distribution of hydraulic conductivity in these sediments must be determined. On a test site (50?×?40 m) directly within the riparian zone of a dredged lake, standard pumping tests, multilevel pumping tests, sieve and sedimentation analyses were carried out. Investigations enabled locating zones with high hydraulic conductivities at specific depths. The hydraulic conductivities determined from each of these methods show a similar trend with increasing depth, but often differ by one order of magnitude.     

Schadstoffmassenströme zwischen Grundwasser, Flussbettsedimenten und Oberflächenwasser am regional kontaminierten Standort Bitterfeld

As a result of intensive industrial, mining, and urban development, numerous large-scale contaminated areas exist in Germany. These so-called megasites represent a challenge to risk assessment and remediation strategies. At the Bitterfeld megasite, the contaminated groundwater interacts with the local streams. Along a stream reach 280?m long, the mass flow rates of chlorinated benzenes were estimated by combining integral pumping tests, streambed temperature mapping, and analyses of contaminant concentrations in the streambed sediments. On average, a total mass flow rate of 617?mg d^–1 monochlorobenzene (MCB) and 157?mg d^–1 dichlorobenzene (DCB) is released from the adjoining aquifer into the stream along the investigated reach. Further, the streambed sediment acts as the dominant contaminant source. Considering the streambed sediments, the contaminant mass flow rate to the river increases to values between 2,355 and 4,323 mg d^–1 MCB and between 892 and 3,587 mg d^–1 DCB.     

南非西格里夸兰盆地某铁锰矿水文地质条件及矿坑涌水量预测

矿坑涌水是矿坑地下开采面临的首要问题,南非西格里夸兰盆地某铁锰矿属于未开采矿山,主要含水层为第四系松散岩类孔隙水含水层和基岩裂隙含水层.第四系松散岩类孔隙水含水层接受大气降水的入渗补给,主要排泄方式为蒸发和向基岩裂隙含水层补给.基岩裂隙含水,主要接受大气降水和第四系松散岩类孔隙水含水层补给,并沿导水性较好的构造裂隙径流...     

Groundwater seepage as a nutrient source to a drainage lake; Lake Mendota, Wisconsin

The flux of groundwater through shallow-water sediments into Lake Mendota was calculated from hydrologic studies and was measured directly with seepage meters at 106 sites around the lake. Groundwater accounted for a substantial amount (around 30%) of the water budget. Pore water in seepage zones was collected for chemical analysis by two methods: (1) dialysis samplers; and (2) direct gentle suction. This pore water, which was assumed to represent seepage inflow, was considerably lower in nitrogen and phosphorus than surface inflow but was higher in phosphorus and lower in nitrogen than well water, indicating that well chemistry does not provide a good indication of the composition of groundwater entering lakes. Calculations indicated that seepage accounted for 12% of the total phosphorus loading to Lake Mendota and 2% of the total nitrogen budget. These results are interpreted in terms of the annual nutrient loading estimates that have been done on Lake Mendota involving only surface water measurements.     

A comparative analysis of iron,manganese, silica,phosphorus, and sulfur in the hypolimnia of calcareous lakes

The chemistry of the hypolimnion was studied in late summer in 23 individual basins of 19 calcareous lakes situated in the Midwestern U.S.A. The basins represented a broad range in morphometry, and drainage basin influences. All but two of the basins were anoxic by early September.The concentrations of Fe were low in the anoxic basins because of precipitation of FeS. Excess sulfide accumulated in the hypolimnion (up to 7 mg 1⁻¹) making these basins sulfuretums. The concentrations of total-P ranged widely—from 0.03 mg 1⁻¹ in several seepage lakes to > 0.5 mg 1⁻¹ in drainage lakes with very short τ_w, or influenced by agricultural or urban drainage, or municipal waste. Neither apatite nor vivianite limited the hypolimnetic build-up of this key nutrient. Hypolimnetic Mn was almost entirely soluble if the basin was anoxic. The concentrations indirectly reflected external inputs, not the thermodynamics of rhodochrosite. Concentrations of dissolved silica were generally much lower than in inflowing waters. The hypolimnetic Si/P ratio was low unless the lake lacked surface inflows and also experienced high rates of groundwater inflow rich in silica and very low in P.Because of the very low hypolimnetic Fe/P and Fe/Si ratios, phosphate and silica are liberated from significant chemical interactions with Fe accompanying their eddy diffusion transport upward within the hypolimnion, and across the metalimnetic oxycline. Because of external P inputs and the efficiency of internal P recycling, Si is potentially a limiting nutrient for diatoms in calcareous lakes.     

Grundwasseranbindung von alten Tagebaurestseen im Niederlausitzer Braunkohlentagebaugebiet

Groundwater with high acidity often leads to strongly acidic residual lakes in the vicinity of abandoned open pit lignite mines. Residual lake 77 (RL77) in Lower Lusatia (Germany) belongs to a sequence of residual lakes accomplishing groundwater drainage. A two-part high resolution three-dimensional groundwater model was set-up for the estimation of groundwater discharge into RL77. The objective of the simulations was to quantify the hydraulic component of the mass balance of the lake. The calculated total discharge could be distinguished with respect to water entering from the dump areas (11.3 l s ^?1 ) and from the undisturbed sediments (3.1 l s ^?1 ). The discharge was further distinguished with respect to the vertical entrance location into the temporarily stratified lake. It showed that groundwater discharged into the epilimnion of the lake at a rate of 9.7 l s ^?1 and into the hypolimnion at a rate of 4.7 l s ^?1 . Both the large fraction of water from the dump areas and the preferential entrance into the oxygen rich epilimnion of the lake have an adverse effect on lake water quality.     

隧道涌水量综合渗透系数的推导及应用

考虑到地下水流经各层的面积、流量和渗透系数的关系,运用地下水动力学理论,推导在隔水底板水平、无限补给和直线补给边界潜水含水层中,非完整型隧道涌水量计算公式的综合渗透系数。用三种隧道涌水量计算方法,分别代入综合和几何平均渗透系数,计算雪峰山隧道两区段涌水量,与实测涌水量进行对比,结果表明:新的综合渗透系数的算法预测隧道涌水量具有一定的实用价值。     

Groundwater seepage nutrient loading in a Florida Lake

Twenty-five seepage meters were positioned in East Lake Tohopekaliga, Florida, to determine groundwater seepage contributions of water and nutrients to the lake in 1983. Seepage was found to be an important source of water to the lake, contributing 14.3% of the water sources, and rates decreased significantly (P < 0.01) with distance from shore. A comparison of the piezometer and seepage meter techniques for measuring nutrient loading to the lake indicates the direct seepage meter technique overestimated nutrient inputs due to the enclosure to the sediments, possibly resulting in anaerobic conditions and increased release rates of ammonium nitrogen and phosphate. These results suggest that past studies employing this technique may be in error. Nutrient loading, calculated from piezometer nutrient data and seepage meter flow data, show that the groundwater nutrient loading in the lake was significant, contributing 8.7 and 17.6% of the total phosphorus and total nitrogen inputs to the lake, respectively.     

Isotopenuntersuchungen zur Wechselwirkung von Grund- und Oberflächenwasser in der Entstehungsphase von Bergbauseen

Environmental isotopes such as ³ H, ² H, ¹⁸ O, ³⁴ S and ²²² Rn were applied to characterize relationships within and between ground and surface waters in two developing mining lakes. Using a two-component-model the portion of saline waters ascending through the lake bottom into the lakes was estimated. Similarly the contribution of pyritic sulphur participating in the lake sulfate has been assessed. Locally, the lakes are hydrochemically stratified. Mixing processes are reflected by all isotopes mentioned above. δ ¹³ C _DIC values of the most mineralized lake water did not correspond with those of deep groundwater from Zechstein strata revailing additional effects such as CO ₂ production. ²²² Rn was checked for assessing groundwater fluxes through the lake bottom. ²²² Rn was found in lake water regions with elevated salt contens but being always in an equilibrium with radium (excepting the sediment-lake water interface). Thus, radium must be taken in consideration in order to interprete radon values in saline groundwaters.     

Ermittlung des Grundwasserdargebotes der Berliner Wasserwerke mittels regionaler numerischer Grundwasserströmungsmodelle

Using available observed and digital data from the hydrogeological systems within the Berlin region, two regional numerical groundwater flow models were developed using a common methodology. These models encompass for the first time an entire area of about 1,300?km² of the groundwater flow system within the common sub-surface catchment area of the Berlin water works. The hydrogeological model and the model aquifers were developed using a unique approach from the available hydrogeological maps and sections of the Geological surveys from Berlin and Brandenburg. The numerical models were calibrated using equipotentials of the pumped aquifer for representative conditions of groundwater extraction as well as using hydraulic information. The models were applied to predict the groundwater yield of eleven water works for the Berlin government water agency.     

Groundwater quality and modelled chemical composition of water in lignite overburden dump aquifers (surface mine Inden, Germany)

The future chemical composition of water in a lignite mining dump aquifer (surface mine Inden II) and in a proposed mine pit lake are modelled. The conceptual geochemical model was verified by means of measured data, which were obtained from a nearby overburden heap. The investigations indicate a decomposition of geogenic pyrites, which is caused by a change in redox conditions and which affects 14?% of the total pyrite content (approx. 0.07 wt.?%). Furthermore, the decomposition results in sulphate concentrations in the groundwater from 1,300 to 1,500 mg/l. The dissolution of calcite and dolomite prevents acidification of the groundwater. Under near-neutral pH-conditions, the dissolution of feldspars is less important and provides only about 1/65 of the alkalinity allocated by carbonate weathering. Precipitation of siderite retains up to 85?% of the liberated ferrous iron, so that iron concentrations in the proposed dump lake should be low and should be accompanied by neutral pH-values.     

Vergleich von hydraulischen und chemischen Sedimenteigenschaften aus Spül- und Kernbohrungen im Raum Peine (Norddeutschland)

Comparison of hydraulic and chemical properties of sediments from flush- and core drillings in the area of Peine (Germany). Because of financial constraints, investigations of nitrate metabolism are often based on disturbed borehole samples. It is arguable, however, whether disturbed samples are suitable for these types of investigations. Disadvantages of disturbed samples in comparison to undisturbed core samples are well known and include possible contamination of the sample by mud additives, destruction of the sediment formation and the insecurity concerning the correct depth allocation. In this study, boreholes were drilled at three locations to a maximum depth of 50?m. The extracted samples, as intact sediment cores and drill cuttings, were studied with regard to chemical and hydraulic parameters of the aquifer sediments. The results show: 1. hydraulic parameters are not affected by clay-based mud; 2. disturbed samples contain less fine grain material relative to the core samples, and the hydraulic conductivity can only be estimated from catch samples; 3. catch samples contain fewer reducing agents (sulphides, organic carbon) than core samples in hydraulically passive zones (defined as K < 10^–6?m?·?s^–1); 4. the results of analyses of disturbed and undisturbed core samples are in good agreement for hydraulically active zones (K ≥ 10^–6?m?·?s^–1).     

Isotopenhydrologische Untersuchungen und Tracerversuche im Zechsteinkarst am Nordrand des Thüringer Waldes

Kurzfassung: Am Nordabhang des Thüringer Waldes werden die Grundw?sser der permotriassischen Sedimentfolgen wasserwirtschaftlich genutzt. Die Sicherung und der Schutz der Fassungsanlagen erfordert die gesicherte Ermittlung des Geschütztheitsgrades sowie der Lage der Einzugs- und Regenerationsgebiete. Die ANwendung isotopen- und tracerhydrologischer Feldmethoden erm?glicht die Erkennung der hydraulischen Verbindungen zwischen den Karst-, Kluft- und Porengrundwasserleitern der Schichtenfolge. Die mittleren Verweilzeiten in den Zechsteinquellen betragen nach den Tritiumgehalten (Exponentialmodell) zwischen 2 und 10 Jahren. Kurzfristige Jungwasserzutritte konnten weder hydrochemisch noch aufgrund der Tritiumgehalte nachgewiesen werden. Eine signifikante Absenkung der δ-Notation in den δ ¹⁸ O-Ganglinien der Quellen und Vorfluter weisen dagegen auf einen kurzzeitigen raschen Zutritt von Jungw?ssern nach Starkregenereignissen aus dem südlichen Regenerationsgebiet (Zechsteinausbi?) hin. Eine Markierung der Zechstein- und Buntsandsteinw?sser mit Farbstoffen und Sole führte zur Abgrenzung von drei hydraulischen Niveaus, die innerhalb des Aquifersysems durch unterschiedliche Quellen drainiert werden. Die dominanten Abstandsgeschwindigkeiten betragen im Zechsteinkarst am Nordoststrand des thüringischen Schiefergebirges 2.400 m/d. Der übertritt von Grundwasser aus dem Porengrundwasserleiter des Unteren Buntsandsteins in den Plattendolomit wurde durch eine mehrgipflige gew?lbte Durchgangskurve best?tigt. Die kombinierte Anwendung verschiedener Tracermethoden erm?glichte in den Fassungen die Unterscheidung kurzfristig zustr?mender W?sser aus unterschiedlichen Einzugsgebieten sowie deren Durchmischung mit dem statischen und dynamischen Vorrat in den Teilaquiferen des Deckgebirges.

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On the orthern rim of the Thuringian forest, groundwaters in the permo-triassic sedimentary aquifer series are utilized on an economic basis. For the securing and protection of the water withdrawal installations a secure determination of the grade of protection as well as the location of the encatchment and regeneration areas is essential. The application of isotope hydrologic and tracer hydraulic field methods enables a distiction of the hydraulic relationship between the karst, fissured and porous aquifers. According to the tritium content (exponential modell), the mean residence time in the Zechstein springs is between 2 and 10 years. Short term recharge of younger groundwater can not be detected by either hydrochemical or tritium methods alone. A significant reduction of the δ-notation in the δ ¹⁸ O-hydrographs of the springs and creeks, however, indicates and abrupt and rapid infiltration of younger groundwater after strong precipitation, originating rom the southern Zechstein outcrop recharge area. A marking of the Zechstein and Bunter sandstone waters with dyes and brine lead to the differentiation of three hydraulic levels, each of which is drained by different springs.On the northeastern verge of the Thuringian forest, the dominant apparent flow velocity within the Zechstein karst is 2.400 m/d. The passage of groundwater from the porous aquifer of the lower Bunter sandstone into the plate dolomites was confirmed by means of a multi-summited, arched transition curve. The combined application of various tracer methods allows the distinction of short-termed infiltration of waters from different catchment areas as well as their mixture with static and dynamic groundwater reserves in the sub-aquifers of the overlying rocks.

     

Hydrogeologisches Konzeptmodell von München: Grundlage für die thermische Grundwassernutzung

The Munich Gravel Plain has been studied under various aspects, but there has been no detailed hydrogeological underground model of the entire city area. As a result of the ??urban heat-island?? effect, groundwater temperatures reach 18?°C and locally exceed 20?°C. This aquifer is therefore suitable for systematic and large-scale near-surface geothermal energy exploitation. The goal of this study was to establish the first detailed hydrogeological model of the city of Munich, as a basis for estimating the geothermal potential. A database of more than 20,000 drill holes was available, of which 730 were selected, interpreted and processed. As a result, 26 east-west cross sections were constructed, showing the geometry and structure of the aquifer and the position of the unconfined water table during low, average and high-water conditions. Based on these profiles and basic hydraulic considerations, a groundwater flow rate of about 3?m³/s was established. Results of this study indicate a high potential for thermal groundwater use and corresponding reductions of heating oil consumption and CO₂ emissions. At the same time, thermal rehabilitation of the overheated urban groundwater could be achieved. The quantification of this geothermal potential and the space-time optimisation of thermal groundwater use for heating and cooling require a detailed analysis of groundwater temperatures and numerical groundwater flow and heat transport modelling.     

ModFlow在超深基坑降水模拟中的应用

利用国际上流行与通用的ModFlow渗流数值模拟计算软件,建立该地块地下水渗流数值模型,并对渗流数值模型进行参数识别,确定微承压含水层的水文地质参数（k和ss）,进而利用该数值模型确定抽水试验期间地下水渗流场的时空分布规律,为今后类似工程的数值模拟提供可靠的依据。     

不考虑开挖扰动影响的隧道涌水量预测模型试验研究

 采用大型施工及运营期矿山法隧道渗流模型试验系统,在不同的注浆圈和初衬渗透系数下,不考虑隧道开挖的扰动影响进行涌水量预测,并以Visual-modflow软件进行数值建模分析,结果表明：两者涌水量结果和规律基本吻合,随注浆圈或初衬的渗透系数减小,其试验平均涌水量值均呈非线性下降,各工况单次试验测试值的离散性也逐渐减小,而降低注浆圈渗透系数对涌水量的影响更为明显;改变注浆圈渗透系数,各工况连续采集涌水量结果呈先减小后增大至逐渐平衡的趋势,而改变初衬渗透系数则呈现涌水量逐渐减小直至平衡的趋势;非扰动开挖使地下水向隧道内汇集,隧道周围水力坡度明显增大,等水位线分布密集,掌子面附近等水位线沿隧道开挖方向凹陷;完善的防水体系(初衬及注浆圈)对保持地下水稳定起到显著作用,可有效减弱局部水力联系,减小渗流影响范围,地下水降幅也明显减小。研究方法及成果对类似工程设计和施工具有一定的指导作用。     

Steinbruchseen—Aufschlüsse im Kluftgrundwasserleiter

Kurzfassung Drei benachbarte Steinbruchseen und die sie umgebenden Grundwasserbeobachtungspegel im Unteren Halleschen Porphyrkomplex (Kautzenberg bei Löbejün) wurden geochemisch untersucht, um die hydrogeologischen Eigenschaften des Gebietes sowie die geochemischen Prozesse beim Eintritt des Grundwassers in einen See besser zu verstehen und hydraulische Verbindungen zwischen den Seen nachzuweisen. Die für die Untersuchung ausgewählten Seen zeichnen sich dadurch aus, dass sie nahezu parallel zur Grundwasserfließrichtung hintereinander aufgereiht liegen. Eine schwache thermische Schichtung des Seewassers wurde nur in den Sommermonaten nachgewiesen. Die starke Durchmischung der Wassersäule ist auf die windexponierte Lage der Seen sowie Tauchaktivitäten zurückzuführen.See- und Grundwasser können anhand chemischer und isotopengeochemischer Parameter unterschieden werden. Die Isotopenverhältnisse ¹⁸O/¹⁶O und ²H/H werden durch Verdunstungsprozesse im See verändert und dienen als geochemische Tracer im Abfluss der Seen, während die Sauerstoff- und Wasserstoffisotope des Grundwassers denen des Niederschlages entsprechen. Weiterhin werden durch seeinterne biologische Prozesse im Seewasser geringere Si- und -Gehalte gefunden als im einströmenden Grundwasser. Die Verteilung der leichten Seltenen Erden im See- und Grundwasser gibt Hinweise auf Fließpfade, die auch den tektonischen Merkmalen des Untersuchungsgebietes entsprechen.

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Quarry lakes as part of a fractured rock aquifer

Abstract In this study, three abandoned adjacent porphyry quarries, filled by rising groundwater are investigated together with surrounding groundwater observation wells. The aim of this study is to characterise the hydrogeological situation of the surrounding fractured rocks and investigate if there are hydraulic connections between the three pit lakes. Furthermore, the interaction between lake and groundwater is studied. The three lakes are situated next to each other in a row and nearly parallel to the main ground-water flow direction from south to north. Meteorological conditions and diving activities lead to thorough mixing of the water in the lakes.Lake and groundwater can be distinguished by their different chemical and isotopic data. The isotope ratios ¹⁸O/¹⁶O and ²H/H are influenced by evaporation and can be used as geochemical tracers for the outflow of the lakes. Biological processes in the lakes resulted in decreased Si and concentrations in the lake water. REE patterns reflect the tectonic features of the drainage basin.

     

Removal of organics in constructed wetlands with horizontal sub-surface flow: a review of the field experience

Constructed wetlands with horizontal sub-surface flow (HF CWs) have successfully been used for treatment various types of wastewater for more than four decades. Most systems have been designed to treat municipal sewage but the use for wastewaters from agriculture, industry and landfill leachate in HF CWs is getting more attention nowadays. The paper summarizes the results from more than 400 HF CWs from 36 countries around the world. The survey revealed that the highest removal efficiencies for BOD₅ and COD were achieved in systems treating municipal wastewater while the lowest efficiency was recorded for landfill leachate. The survey also revealed that HF CWs are successfully used for both secondary and tertiary treatment. The highest average inflow concentrations of BOD₅ (652 mg l^− 1) and COD (1865 mg l^− 1) were recorded for industrial wastewaters followed by wastewaters from agriculture for BOD₅ (464 mg l^− 1) and landfill leachate for COD (933 mg l^− 1). Hydraulic loading data reveal that the highest loaded systems are those treating wastewaters from agriculture and tertiary municipal wastewaters (average hydraulic loading rate 24.3 cm d^− 1). On the other hand, landfill leachate systems in the survey were loaded with average only 2.7 cm d^− 1. For both BOD₅ and COD, the highest average loadings were recorded for agricultural wastewaters (541 and 1239 kg ha^− 1 d^− 1, respectively) followed by industrial wastewaters (365 and 1212 kg ha^− 1 d^− 1, respectively). The regression equations for BOD₅ and COD inflow/outflow concentrations yielded very loose relationships. Much stronger relationships were found for inflow/outflow loadings and especially for COD. The influence of vegetation on removal of organics in HF CWs is not unanimously agreed but most studies indicated the positive effect of macrophytes.     

Migration of As, and H/He ages, in groundwater from West Bengal: Implications for monitoring

From 2002 to 2010 inclusive we monitored concentrations of arsenic (As) and major ions (Ca, Mg, Sr, Na, K, Fe, Mn, Cl, and SO₄) in groundwater from 14 domestic wells and three piezometer nests in a shallow aquifer (<60 m depth), and 3 wells in a deep aquifer (>70 m depth), in southern West Bengal, India. In the deep aquifer, concentrations of As did not change over time despite increases in the concentration of Fe in two wells. The shallow aquifer occurs in two sedimentological settings: palaeo-channel and palaeo-interfluve. At the top of the shallow aquifer of the palaeo-channel, decreases in all constituent concentrations with time, and an ³H/³He age of 1.4 years, proves that the aquifer is beginning to be flushed of pollutants. In As-polluted groundwater (>50 μg/L As) tapped from deeper grey sands of the shallow, palaeo-channel, aquifer, concentrations of As were mostly stable over time, but both increases and decreases occurred with time in response to downward migration of the chemically-stratified water column. In groundwater tapped from Pleistocene brown sands, the concentration of As remained either low and stable (<2 μg/L As), or increased at rates up to 34 μg/L per year. The increases were caused by the flow of As-rich groundwater either downward into brown sand at the base of palaeo-channels, or laterally into a confined, unpolluted, palaeo-interfluvial, aquifer of brown sand that lies regionally beneath a palaeosol.Under the present pumping regime, the prognosis for As-pollution in the shallow aquifer is complex. Wells in brown sand may become polluted over timescales of as little as 2 years, whilst some wells tapping As-polluted groundwater from grey sand will become fit for potable use (<50 μg/L) within a few decades. The evidence of flushing, and of declining As in some of the groundwater from palaeo-channels, which are conduits for recharge of the confined, As-free, palaeo-interfluve aquifer, and probably also the deeper aquifer, offers hopes that the spread of As-pollution will be limited.