荆州市地下水动态特征及影响因素分析

利用2008~2011年4年荆州地区36个水位观测孔、30个水质监测孔的连续监测资料,结合地下水补径排条件,分析了该区上部孔隙承压水水位和水质的时空变化特征及其影响驱动因子。结果显示:(1)荆州地下水位动态类型分为自然动态型和人为动态型两种,不同类型地下水水位动态规律在时空分布上有明显差异,呈现出周期性、同步性、特殊性等特点。4167%~50%的监测孔地下水位处于稳定状态,2222%~3611%的监测孔地下水位下降幅度大于02 m,1944%~3611%的监测孔地下水位上升幅度大于02 m,地下水位动态主要受气候条件与人工开采条件影响。(2)荆州地区地下水质不容乐观,整体较差。连续四年均超标的指标有总硬度、亚硝酸盐、氨氮、砷、铁、锰、溶解性固体含量,同时在部分监测点存在超标异常情况,铁、锰、砷的超标与江汉平原区域特有的原生地质环境条件密切相关,而其它组分的超标在很大程度上受到人类活动的影响     

Potential for the desalination of a brackish groundwater aquifer under a background of rising sea level via salt intrusion prevention river gates in the coastal area of the Red River Delta,Vietnam

Additional freshwater sources are required in many parts of the world, including the coastal areas of the Red River Delta (RRD), where the groundwater (GW) is generally brackish. Determining a feasible method for desalinating brackish aquifers would help provide additional freshwater sources. However, substantial desalination of brackish aquifers cannot be achieved under the natural conditions of GW flow and precipitation recharge. Although rainfall recharge to the shallow Holocene aquifer has occurred for hundreds of years, the aquifer still remains brackish since the natural hydraulic conditions do not allow a complete mixing between the fresh recharged water and aquifer salinized water or the discharging of the aquifer salinized water. The planned salt intrusion prevention gates in the Red River, Tra Ly River and Hoa River in the RRD coastal area, combined with increased GW abstraction and associated aquifer recharge with fresh river water, could result in the gradual desalination of the shallow Holocene aquifer. These effects would help improve the area’s resilience to freshwater shortages and sea level rises and would allow for the creation of a long-term sustainable water resource development plan to manage the salinization of water resources caused by sea level rises. Finite element (FE) modeling of GW flow, solute transport via GW flow and dynamic programming (DP) have been used to study the potential desalination of brackish aquifers, the magnitude of GW abstraction quantities and the spatial and temporal aspects of desalination. FE modeling of GW flow coupled with DP was utilized to identify the magnitude of sustainable abstraction quantities and the GW flow field, which is required in salt transport models. Multiple sizes of elements and time steps were used to adapt to the unsteady state of GW flow and hydraulic head variables between the elements in the FE meshes in order to ensure reasonable accuracy of numerical modeling. The GW flow and salt transport modeling and DP allowed determining quasi-steady-state GW abstraction rates and aquifer salinity levels for conditions that did and did not include the shallow Holocene unconfined aquifer recharge from rainfall. The aquifer modeled domain which is supposed to serve the pumping well field is 1.5 km². The results showed that the Holocene aquifer may provide a stable abstraction rate of 100 m³/day starting in the 6th year (for the worst-case scenario with zero aquifer recharge from rainfall) to 130 m³/day starting in the 3rd year (for the scenario with aquifer recharge equal to 3% of the rainfall levels). During the first years of GW abstraction, the desalination of the brackish upper Holocene aquifer will mainly occur in the area close to the river, and at the 18th year of abstraction, almost the entire area between the river and line of pumping wells would be desalinized. From the 10th year of abstraction, the abstracted water has a total dissolved solids content lower than 0.5 g/l for the worst-case scenario with zero aquifer recharge from rainfall and lower than 0.42 g/l for the scenario with aquifer recharge equal to 3% of the rainfall. The modeling results indicate the simulated process by which abstraction of groundwater adjacent to the Tra Ly River could desalinize the brackish aquifer via freshwater recharge from the river.     

Delineation of groundwater contamination around an ash pond: geochemical and GIS approach

The study has investigated the levels of metal contamination in groundwater due to particulate matter fallout and leaching from ash pond and assigned contamination indices for the adjacent localities around an ash disposal site with application of geographic information systems (GIS). Fe, Ba, Cu, Mn, S, Pb, V, and Zn were found to be the major contaminants in groundwater. Enrichment factors (EF) of these elements with respect to the United States Environmental Protection Agency (USEPA) maximum contaminant levels show high values for Mn, Fe, and Pb in groundwater. The zone of attenuation for Ba, Fe, Cu, Mn, S, and Zn in groundwater is about 600-900 m from the ash pond, while Pb did not show any significant attenuation even at a distance of 1200 m. Tube wells around Rankasingha and Kukurhanga villages are most contaminated whereas open wells of Lachhmanpur, Kaniapada, and Kurudul villages showed higher degrees of contamination.     

Krypton-85 in the atmosphere 1950–1977: a data review

For over 20 years the radioactive noble gas ⁸⁵Kr, the product of nuclear industry, has been released to the environment mainly from nuclear fuel reprocessing plants. Owing to its relatively long half-life $\text{T(}\text{1}\text{2}\text{= 10.76}\text{y}\text{)}$ and the absence of other appreciable sinks, a cumulative buildup of ⁸⁵Kr in the atmosphere is observed. The mean ⁸⁵Kr surface air activity in the northern hemisphere has increased from about 20 dpm/mmol Kr in 1950 to about 750 dpm/mmol Kr in 1977. In recent years, a substantial slow-down of the ⁸⁵Kr emission rate to the atmosphere has been observed. The published data indicate rather good mixing of ⁸⁵Kr in the northern hemisphere, but a gradient is observed in the southern hemisphere due to the fact that all nuclear fuel reprocessing plants are situated in the northern hemisphere and due to poor mixing of the atmosphere in the meridional direction.This paper summarizes nearly all of the data on ⁸⁵Kr activity in the atmosphere published up to the middle of 1978. The spatio-temporal distribution of ⁸⁵Kr atmospheric activity is discussed. Some forecasts of future ⁸⁵Kr concentration in the atmosphere and its possible ecological consequences are also presented.     

An improved method for Kr analysis by liquid scintillation counting and its application to atmospheric Kr determination

Atmospheric ⁸⁵Kr concentration at Fukuoka, Japan was determined by an improved ⁸⁵Kr analytical method using liquid scintillation counting (LSC). An average value of 1.54 ± 0.05 Bq m⁻³ was observed in 2008, which is about two times that measured in 1981 at Fukuoka, indicating a 29 mBq y⁻¹ rate of increase as an average for these 27 years. The analytical method developed involves collecting Kr from air using activated charcoal at liquid N₂ temperature and purifying it using He at dry ice temperature, followed by Kr separation by gas chromatography. An overall Kr recovery of 76.4 ± 8.1% was achieved when Kr was analyzed in 500-1000 l of air. The Kr isolated by gas chromatography was collected on silica gel in a quartz glass vial cooled to liquid N₂ temperature and the activity of ⁸⁵Kr was measured with a low-background LS counter. The detection limit of ⁸⁵Kr activity by the present analytical method is 0.0015 Bq at a 95% confidence level, including all propagation errors, which is equivalent with ⁸⁵Kr in 1.3 l of the present air under the analytical conditions of 72.1% counting efficiency, 0.1597 cps background count rate, and 76.4% Kr recovery.     

A study on mountain front recharge by using integrated techniques in the hard rock aquifers of southern India

Mountain front recharge (MFR) is the contribution of mountains to recharge the aquifers in the adjacent basins. The estimation of MFR is essential to obtain a detailed investigation of recharge of the groundwater at the mountain front. This study summarises the current understanding of recharge processes by comparing daily groundwater fluctuation to daily rainfall and identifies the recharge rates. The recharge rates vary with time due to difference in water table depth and travel time. Thus to understand the MFR along the foothills of Courtallam, a total of 14 surface water, rainwater and groundwater samples were collected and measured for stable isotopes. The isotopic data were used to investigate the recharge process and to identify the elevations to recharge. The study findings also suggest that predominantly rainfall along the foothills contributes recharge to the riparian zone (basin block region), whereas foothill regions receive recharge from rainfall over mountain block.     

A performance estimate for the detection of undeclared nuclear-fuel reprocessing by atmospheric 85Kr

To test the sensitivity of using atmospheric (85)Kr to detect undeclared separation of plutonium from irradiated nuclear-reactor fuel, measurements of atmospheric (85)Kr taken in Tsukuba, Japan are analyzed to determine: (1) a lower limit of detection for discovering anthropogenic (85)Kr emissions, (2) the probability of detecting plutonium separation at the Tokai Reprocessing Plant, and (3) the extent to which these results can be generalized to other sites. A LLD of at least 3.4 sigma=0.14 Bq/m(3) with a theoretical false-positive rate of 0.05% is recommended for safeguards' purposes. At this threshold, the continuous separation of 100, 300, and 900 g equivalent weapon-grade plutonium per day was found to correspond to 10%, 50%, and 80% probability of detection, respectively. The smallest detected concentration was for the continuous separation of 45 g/day, with a probability of detection of about 0.6%. It was found that the detection rate is determined predominantly by the weather.     

Assessing field-scale migration of radionuclides at the Nevada Test Site: "mobile" species

Many long-lived radionuclides are present in groundwater at the Nevada Test Site (NTS) as a result of 828 underground nuclear weapons tests conducted between 1951 and 1992. In conjunction with a comprehensive geochemical review of radionuclides ((3)H, (14)C, (36)Cl, (99)Tc and (129)I) that are presumably mobile in the subsurface, we synthesized a body of radionuclide activity data measured from groundwater samples collected at 18 monitoring wells, to qualitatively assess their migration at the NTS over distances of hundreds of meters and over timescales of decades. Tritium and (36)Cl showed little evidence of retardation, while the transport of (14)C may have been retarded by its isotopic exchange with carbonate minerals in the aquifer. Observed local reducing conditions (either natural or test-induced) will impact the mobility of certain redox-sensitive radionuclides (especially (99)Tc) that were otherwise soluble and readily transported under oxidizing conditions. Conversely, strongly oxidizing conditions may impact the mobility of (129)I which is mobile under reducing conditions. The effect of iodine speciation on its transport deserves further attention. Indication of delayed transport of some "mobile" radionuclides (especially (99)Tc) in the groundwater at the NTS suggested the importance of redox conditions of the natural system in controlling the fate and transport of radionuclides, which has implications in the enhanced performance of the potential Yucca Mountain repository, located adjacent to the NTS, to store high-level nuclear wastes as well as management of radionuclide contamination in legacy nuclear operations facilities.     

Spatial variability of shallow groundwater level, electrical conductivity and nitrate concentration, and risk assessment of nitrate contamination in North China Plain

In recent years, nitrate (NO3) contamination of groundwater has become a growing concern for people in rural areas in North China Plain (NCP) where groundwater is used as drinking water. The objective of this study was to evaluate groundwater resource level, to determine groundwater quality and to assess the risk of NO3 pollution in groundwater in Quzhou County in the NCP. Ordinary Kriging (OK) method was used to analyze the spatial variability of shallow groundwater level, groundwater electrical conductivity (EC) and NO3-N concentrations, and Indictor Kriging (IK) method was used to analyze the data with NO3-N concentrations equal or greater than the groundwater NO3 pollution threshold (20 mg L(-1)). The results indicated that groundwater level averaged 9.81 m, a level 6 m lower than in 1990. The spatial correlation distances for groundwater level, EC and NO3-N concentration were 21.93, 2.19 and 3.55 km, respectively. The contour map showed that shallow groundwater level areas extended from north to south across the County. Groundwater EC was above 3 dS m(-1) in the most part of the northern county. Groundwater NO3 pollution (NO3-N> or =20 mg L(-1)) mainly occurred in the County Seat areas due to wastewater irrigation and excessive fertilizer leaching from agricultural fields. At Henantuang town, besides suburban of the County Seat, groundwater was also contaminated by NO3 shown by the map generated using the IK method, which was not reflected in the map generated using the OK method. The map generated using the OK method could not reflect correctly the groundwater NO3 pollution status. The IK method is useful to assess the risk of NO3 pollution by giving the conditional probability of NO3 concentration exceeding the threshold value. It is suggested that risk assessment of NO3 pollution is useful for better managing groundwater resource, preventing soil salinization and minimizing NO3 pollution in groundwater.     

A new compilation of the atmospheric 85krypton inventories from 1945 to 2000 and its evaluation in a global transport model

This paper gives the yearly (85)Kr emissions of all known reprocessing facilities, which are the main sources of (85)Kr in the atmosphere since 1945, for the years 1945 until 2000. According to this inventory 10,600 PBq (Peta=10(15)) of (85)Kr have been globally emitted from the year 1945 until the end of 2000. The global atmospheric inventory at the end of the year 2000 amounts to 4800 PBq. These emissions have been incorporated into the ECHAM4 atmospheric general circulation model as point sources. Monthly mean model results are compared with measurements made at different locations and times. The influence of each source on the measured concentrations at various locations is studied. The calculated concentrations are found to give reasonably good agreement with the observations, indicating that the emission inventory is realistic. Although, at all northern hemispheric observation sites the model tends to slightly overestimate the concentrations. A possible reason for this overestimation can be found in model features (coarse resolution in time and space). The most prominent discrepancy that is consistently repeated at all northern hemispheric stations occurs in the early 1990s. This could most likely be related to an overestimate of sources. Possibly, the Russian emissions declined earlier than assumed in the current database. Another discrepancy between observations and simulations indicating an incompleteness of the release data is found at some southern hemispheric sites. The variability of their observations could only be explained by regional sources. However, several spikes occur after 1992 when no reprocessing facility is known to be in operation in the southern hemisphere. Production of isotopes for radiopharmaceuticals like technetium-99m from highly enriched uranium is the most likely explanation.     

Radium geochemistry in Na-Cl type groundwater in Niigata Prefecture, Japan

Radium isotopes in 23 Na-Cl type groundwater sampled mainly from deep wells in Niigata Prefecture, which is the site of the largest oil- and gas-fields in Japan, were measured along with U isotopes, chemical components and hydrogen and oxygen isotope ratios to elucidate the distribution and behavior of Ra in a brackish environment underground. Also analyzed were U and Th isotopes in 38 rock samples collected from outcrops at 17 locations. Ra-226 concentrations (8.86-1637 mBq kg⁻¹) of groundwater samples roughly correlated with total dissolved solid (TDS) concentrations and other alkaline earth contents. Their ²²⁸Ra/²²⁶Ra activity ratios (0.32-5.2) were similar to or higher than the ²³²Th/²³⁸U activity ratios (0.6-1.7) in the rocks. The most likely transport mechanism of Ra isotopes into groundwater was due to their α-recoil from the solid phase, probably from the water-rock interface where Th isotopes had accumulated, and adsorption/desorption reaction based on the increase in ²²⁶Ra contents with TDS.     

Climate change and modeling of an unconfined aquifer: the Triffa plain,Morocco

The Triffa plain covering about 307 km² is located in the semiarid region of northeastern Morocco. The cover consists of Quaternary and Mio-Pliocene formations, including alluvial material, silt, sandstone, limestone, clay, and marl, underlain by a sequence of Jurassic carbonates and clastics. Two principal aquifers occur in this region: (1) An unconfined aquifer hosted by the Quaternary formations, which opens up on the coastal plain of Saïdia, giving rise to the Aïn Chebbak and Aïn Zebda springs; and (2) a confined aquifer hosted by the underlying Liassic (Jurassic) formations, composed of limestone and dolostone. In this paper, we present a conceptual hydrogeological model for the Triffa aquifer, which opens laterally into the Saïdia aquifer, based on borehole data, bedrock geology, hydrodynamic parameters, piezometric maps, and time series groundwater level and precipitation data, obtained from several meteorological stations and pumping wells. These comprehensive data were incorporated in the Geographic Information System platform and processed using groundwater modeling software, with the development of the numerical model and its limitations discussed in detail in the present work. Subsequently, we evaluated the impact of climate change on the Triffa aquifer, assuming three different climate scenarios developed by the Intergovernmental Panel on Climate Change. These were the B1—low, A1B—mean, and A1F1—extreme scenarios, which we modeled by simulating a decrease in the recharge in all of the study area up to year 2099 that would correspond to 9, 19, and 47%, respectively. The calibration of the model in steady and transient states produced a good agreement between the observed and simulated heads. The simulation of the impact of climate change on groundwater by a decrease in the recharge highlights the groundwater drawdown occurring in this region. This work can significantly help the authorities in the sustainable management and exploitation of local groundwater.     

川中遂宁地区红层地下水水化学特征及主要离子来源分析

四川红层区居民用水主要为就近开采的地下水,水质参差不齐,研究红层地下水水文地球化学特征,对于宜井区选择,红层水源地用水安全具有重要意义。利用在遂宁船山区采集的70组浅层地下水水样,采用数理统计分析、离子比例关系等方法,分析了红层浅层地下水水化学特征及主要离子来源,研究得出:区内地下水中阴离子以HCO_3~-(164.00～571.91 mg·L~(-1))为主,其次是SO_4~(2-)(13.96～1 000.00 mg·L~(-1)),阳离子以Ca~(2+)(45.70～313.00 mg·L~(-1))为主,其中孔隙水的水化学类型主要为HCO_3,SO_4-Ca,SO_4-Mg型,裂隙水以HCO_3-Ca,Mg型与HCO_3-Ca型为主;地下水中离子成分主要由岩石风化产生,来源于地层中的方解石、白云石、石膏、芒硝等矿物的风化溶解,蒸发浓缩、地下水酸碱环境及人类活动对地下水成分造成了一定的影响;浅层地下水质量以Ⅲ～Ⅳ类为主,污染物主要为TDS、Fe~(3+)等原生污染,也存在NO_3~-、NO_2~-等人类活动污染;红层宜井区主要为平坝、丘坡坡脚及沟谷一带,为避免污染指标超标,井深不宜超过25m,远离养殖场、农厕等地下水易受人类活动污染的环境,防止鱼塘、农田等地表水进入井中。     

Economic implications of groundwater exploitation in hard rock areas of southern peninsular India

The present paper analyses the consequences of groundwater exploitation by using field-level data collected from two distinct well irrigated areas of Karnataka. The study results show that the consequences arising out of groundwater overexploitation are severe in high well interference area compared to low well interference area. The burden of well failure is more or less equally shared by all categories of farmers but small farmers are the worst victims of resource scarcity. As a result, overexploitation of groundwater has different impacts on different categories of farmers in terms of access to groundwater, cost and returns to groundwater irrigation and its negative externality cost. The study suggests maintaining inter-well distance to prevent resource mining and calls for supply and demand side interventions. The institutional reform is necessary to restore surface water bodies to facilitate aquifer recharge.     

上海城区地面沉降及其对地下水采灌量的响应

上海城区地面沉降主要是由超采地下水、松散沉积地层压实等因素共同作用导致的,并引发多种次生灾害。F4分层标监测数据指示的地面沉降情况,显示上海城区地面沉降表现为总体具有非线性变化的规律,特别是1980年代以来,表现出垂向逐渐增大的特点,在空间上也不断扩展,直至2001年才始有缓和的趋势。在不同时段,随地下水开采量及开采层次的变化,上海城区地面沉降表现出不同的特征。结合1990~2009年20年内第四含水层地下水采灌量数据,得出地面沉降对地下水采灌量的响应关系为:地下水开采量增加,地层沉降量增大;地下水开采量减少,地层沉降量减小,但地面沉降相对于地下水开采具有滞后性;随着逐年的回灌,地层沉降量减小,且地下水回灌效果与回灌层位有直接关系。     

85Kr atmospheric concentration in Bratislava from 1980 to 1983

A cryogenic-chromatographic method for determination of ⁸⁵Kr in the atmosphere is presented. Sampling of the air on the activated-charcoal, chromatographic separation of Krypton from the other air components and radioactivity measurement are described in details. The activity of ⁸⁵Kr is measured by small proportional counters with sensitive volumes of 5 and 10 cm³. Results of the measurements of ⁸⁵Kr air concentration in Bratislava from 1980 to 1983 are presented and compared with other ⁸⁵Kr data measured in Europe.     

Simulating mesoscale transport and diffusion of radioactive noble gases using the Lagrangian particle dispersion model

In order to simulate the impact of mesoscale wind fields and to assess potential capability of atmospheric Lagrangian particle dispersion model (LPDM) as an emergency response model for the decision supports, two different simulations of LPDM with the mesoscale prognostic model MM5 (Mesoscale Model ver. 5) were driven. The first simulation of radioactive noble gas ((85)Kr exponent) emitted during JCO accident occurred from 30 September to 3 October 1999 at Tokai, Japan showed that the first arriving short pulse was found in Tsukuba located at 60km away from the accidental area. However, the released radioactive noble gas was transported back to the origin site about 2 days later due to the mesoscale meteorological wind circulation, enhancing the levels of (85)Kr with the secondary peak in Tsukuba. The second simulation of atmospheric dilution factors (the ratio of concentration to the emission rate, chi/Q), during the underground nuclear test (UNT) performed by North Korea showed that high chi/Q moved to the eastward and extended toward southward in accordance with the mesoscale atmospheric circulations generated by mesoscale prognostic model MM5. In comparison with the measurements, the simulated horizontal distribution patterns of (85)Kr during the JCO are well accord with that of observation in Tsukuba such as the existence of secondary peak which is associated with the mesoscale circulations. However, the simulated level of (85)Kr anomaly was found to be significantly lower than the observations, and some interpretations on these discrepancies were described. Applications of LPDM to two mesoscale emergency response dispersion cases suggest the potential capability of LPDM to be used as a decision support model provided accurate emission rate of accident in case of a large accident.     

近15年来长江口控制站徐六泾悬沙变化特征研究

基于近15年的实测数据,分析给出了长江河口控制站徐六泾悬沙浓度和组成的变化特性。研究表明：徐六泾站多年平均表层（2003～2017年）、中层和底层（2010～2017年）含沙量分别为0.07、0.07和0.13 g/L;表层（2004～2017年）、中层和底层（2009～2017年）的中值粒径分别为7.3、9.4和11.0 μm。研究期间,年平均含沙量呈显著下降趋势,洪季较枯季降幅更明显。其中2015～2017年均洪枯季表层含沙量较2003～2005年均值分别降低了57%和20%;表层含沙量在2008年出现明显降低的拐点,2008年之后的洪枯季表层含沙量较之前分别降低56%和44%。悬沙粒度具有一定的波动变化,但没有显著趋势性变化特征,或与样本数量限制和流域内人类活动有关。     

Integrated hydrologic–economic decision support system for groundwater use confronting climate change uncertainties in the Tunuyán River basin,Argentina

This study presents an integrated hydrologic–economic model as decision support system for groundwater use and incorporates uncertainties of climate change. The model was developed with the Vensim software (Ventana Systems) for system dynamic simulations. The software permitted the integration of economic variables along with hydrologic variables, in a unified format with the aim of evaluating the economic impacts of climate change on arid environments. To test the model, we applied it in one of the upper Tunuyán River sub-basin, located in the Mendoza Province (Argentina), where irrigation comes from groundwater. The model defines the best mix of crops and the total land use required to maximize the total river sub-basin monetary income, considering as a limit the amount of water that does not exceed the natural annual aquifer recharge. To estimate the impacts of climatic changes, four scenarios were compared: the business as usual (with the number of existing wells) in a dry year with a temperature increase of 4 °C; the business as usual in a wet year with an increase in temperature of 1.1 °C; an efficient use of wells in a dry year and a temperature increase of 4 °C and an efficient use of wells in a wet year with a temperature increase of 1.1 °C. Outputs calculated by the model were: land use per crop, total sub-basin net benefit, total sub-basin water extraction, water extraction limit depending on river discharge and total number of wells required to irrigate the entire area. Preliminary results showed that the number of existing wells exceeded the optimized number of wells required to sustainably irrigate the entire river sub-basin. Results indicated that in an average river discharge year, if wells were efficiently used, further rural development would be possible, until the limit of 350 million m³ of water extraction per year was reached (650 million m³ for a wet year and 180 million m³ for a dry year). The unified format and the low cost of the software license make the model a useful tool for Water Resources Management Institutions, particularly in developing countries.     

Estimation of septic tank setback distances based on transport of E. coli and F-RNA phages

Setback distances between septic tank systems and the shorelines of Lake Okareka, New Zealand were determined from model simulations for a worst-case scenario, using the highest hydraulic conductivity and gradient measured in the field, removal rates of the microbial indicators (Escherichia coli and F-RNA phages) determined from a column experiment, and maximum values of the design criteria for the disposal system, and assuming an absence of an unsaturated zone, a continuous discharge of the raw effluent from a failed or non-complying treatment system (both indicators at concentrations of 1x10(7) counts/100 ml) into the groundwater and no sorption of pathogens in the aquifer. Modelling results suggest that the minimal setback distances were 16 m to satisfy the New Zealand Recreational Water Quality Guidelines for E. coli <126 per 100 ml (Ministry for the Environment, 1999) and 48 m to meet the Drinking-Water Standards for New Zealand 2000 for enteric virus <1 per 100 l (Ministry of Health, 2000). These distances may be applicable for other lakeshores in pumice sand aquifers with groundwater velocities <7 m/day. Findings of laboratory column and batch experiments provided an insight into the microbial attenuation and transport processes in pumice sand aquifers. Bacterial removal was predominately through filtration (87-88%) and partially by die-off (12-13%), while viral removal was by both die-off (45%) and filtration (55%). In addition, microbial die-off in groundwater without aquifer material (i.e., free microbes) was much lower than die-off in groundwater with aquifer material (i.e., sorbed microbes) and contributed only 2-6% to the total removal. This implies that the setback distances estimated from die-off rates for the free microbes, determined in the laboratory without considering aquifer media and other removal processes, which are often reported in the literature, could be larger than necessary.