Bootstrap confidence intervals for design storms from exceedance series

Abstract

An approach for better understanding of the physical implication of estimated aquifer parameters is demonstrated by analysing the pumping test data at Cottam in the Nottingham aquifer, UK. A sensitivity analysis showed that the area represented by the estimated parameters was much smaller than the area covered by the depression cone. After parameters are estimated, further research should be carried out to understand what portions of the aquifer the parameters represent. The parameters estimated at Cottam represented mainly aquifer features between roughly 100 and 2000 m. The sensitivity analysis also showed that the observed drawdown being satisfactorily matched by a model with uniform parameters does not prove that the aquifer is homogeneous. Slightly anomalous data may imply the existence of large anomalous zones. Although the drawdowns at Cottam could be ‘satisfactorily’ fitted by a model with uniform parameters, the fit could be improved by a model using a more permeable aquifer but with a zone about 700 m wide and with 42% less transmissivity.     

Isotopic tracing for conceptual models of groundwater hydrodynamics in multilayer aquifer systems of central and southern Tunisia

Abstract

New and previously published data sets including stable and radiogenic isotope measurements (¹⁸O, ²H, ³H, ¹³C and ¹⁴C) were used to investigate, conceptualize and compare groundwater hydrodynamics within three major multilayer aquifer systems located in central and southern Tunisia. It has been demonstrated that the investigated aquifer systems contain modern and palaeoclimatic waters. Modern groundwaters, which refer to contemporaneous and post-nuclear recharge waters, are characterized by enriched stable isotope contents, high carbon-14 activities and high to moderate tritium concentration. While palaeoclimatic groundwaters, which refer to Late Pleistocene and Early Holocene recharge waters, are distinguished by their depleted stable isotope contents, low carbon-14 activities and insignificant tritium concentrations. Established conceptual models have elucidated the groundwater hydrodynamics within the studied aquifer systems. They show that groundwater mixing occurs between end-members from the shallow and deep aquifers that migrate by downward and upward leakage towards the intermediate aquifer.

Editor D. Koutsoyiannis; Associate editor S. Faye

Citation Dassi, L. and Tarki, M., 2014. Isotopic tracing for conceptual models of groundwater hydrodynamics in multilayer aquifer systems of central and southern Tunisia. Hydrological Sciences Journal, 59 (6), 1240–1258. http://dx.doi.org/10.1080/02626667.2014.892206     

Deciphering groundwater flow between the Complex Terminal and Plio-Quaternary aquifers in Chott Gharsa plain (southwestern Tunisia) using isotopic and chemical tools

Abstract

The Complex Terminal (CT) and Plio-Quaternary (P-Q) aquifers in the Chott Gharsa plain in southwestern Tunisia have been investigated with the aid of chemical and isotopic tools. It has been demonstrated that groundwater from the CT is mainly of palaeo-origin, especially in the western and central parts of the plain where the most negative values of δ¹⁸O and δ²H were observed (between??8.1 and??7.6‰ for δ¹⁸O, and??60 to??57‰ for δ²H), combined with low concentrations of radiocarbon (6.8–7.5 pmc) and absence of tritium. Modern recharge of the aquifer occurs only in the eastern part of the system where younger waters were observed, as indicated by their stable isotope composition, relatively high radiocarbon content and presence of tritium. Groundwater from the P-Q multi-layer aquifer represents mixtures of ascending deep CT waters and modern water recharging the P-Q aquifer system. Isotope mass balance was used to quantify mixing proportions. The calculations showed that the contribution of deep CT groundwater to the P-Q aquifer system reaches about 75% in the western and central parts of the plain where the CT aquifer remains strongly artesian. This contribution decreases to about 15% towards the eastern part of the plain, as a consequence of significant reduction of artesian pressure in this area of the CT aquifer. Chemical data suggest that mineralization of the studied groundwater systems is controlled mainly by dissolution of evaporative minerals (halite, anhydrite and gypsum) and cation exchange reactions with the matrix, possibly enhanced by recent anthropogenic disturbance of the system caused by lowering of the water table due to heavy exploitation and return flow of saline irrigation water into the P-Q aquifer.

Editor D. Koutsoyiannis; Associate editor E. Custodio

Citation Yangui, H., Abidi, I., Zouari, K., and Rozanski, K., 2012. Deciphering groundwater flow between the Complex Terminal and Plio-Quaternary aquifers in Chott Gharsa plain (southwestern Tunisia) using isotopic and chemical tools. Hydrological Sciences Journal, 57 (5), 967–984.     

Soil profile evolution following land‐use change: implications for groundwater quantity and quality

Soil and vadose zone profiles are used as an archive of changes in groundwater recharge and water quality following changes in land use in an area of the Loess Plateau of China. A typical rain‐fed loess‐terrace agriculture region in Hequan, Guyuan, is taken as an example, and multiple tracers (chloride mass balance, stable isotopes, tritium and water chemistry) are used to examine groundwater recharge mechanisms and to evaluate soil water chloride as an archive for recharge rate and water quality. Results show that groundwater recharge beneath natural uncultivated grassland, used as a baseline, is about 94–100 mm year^?1 and that the time it takes for annual precipitation to reach water table through the thick unsaturated zone is from decades to hundreds of years (tritium free). This recharge rate is 2–3 orders of magnitude more than in the other semiarid areas with similar annual rainfall but with deep‐rooted vegetation and relatively high temperature. Most of the water that eventually becomes recharge originally infiltrated in the summer months. The conversion from native grassland to winter wheat has reduced groundwater recharge by 42–50% (50–55 mm year^?1 for recharge), and the conversion from winter wheat to alfalfa resulted in a significant chloride accumulation in the upper soil zone, which terminated deep drainage. The paper also evaluates the time lag between potential recharge and actual recharge to aquifer and between increase in solute concentration in soil moisture and that in the aquifer following land‐use change due to the deep unsaturated zone. Copyright © 2012 John Wiley & Sons, Ltd.     

Ecohydrologic considerations for modeling of stable water isotopes in a small intermittent watershed

Naturally occurring stable water isotope tracers provide useful information for hydrologic model development and calibration. Existing models include varied approaches concerning unsaturated zone percolation mixing (preferential versus matrix flow) and evapotranspiration (ET) partitioning. We assess the impact of unsaturated zone simplifying assumptions when simulating the Shale Hills Watershed, a small (7.9 ha), temperate, forested watershed near Petersburg, Pennsylvania, USA, with a relatively simple model. We found that different model structures/assumptions and parameterizations of unsaturated zone percolation had substantial impacts on the agreement between simulated and observed unsaturated‐zone water isotopic signatures. We show that unsaturated zone percolation mixing primarily affects the unsaturated zone δ¹⁸O and δ²H during winter and spring and that percolation was best represented as a combination of both preferential and matrix flow. We evaluate the importance and implications related to the partitioning of ET into evaporation and transpiration and demonstrated that incorporation of a plant growth model for ET partitioning substantially improved reproduction of observed hydrologic isotopic patterns of the unsaturated zone during the spring season. We show that unsaturated zone percolation mixing and ET partitioning approaches do not substantially influence stream δ¹⁸O and δ²H and conclude that observed streamflow isotopic data is not always a strong predictor of model performance with respect to intrawatershed processes.     

Groundwater recharge in the Akaki catchment,central Ethiopia: evidence from environmental isotopes (δ18O, δ2H and 3H) and chloride mass balance

Recharge patterns, possible flow paths and the relative age of groundwater in the Akaki catchment in central Ethiopia have been investigated using stable environmental isotopes δ¹⁸O and δ²H and radioactive tritium (³H) coupled with conservative chloride measurements. Stable isotopic signatures are encoded in the groundwater solely from summer rainfall. Thus, groundwater recharge occurs predominantly in the summer months from late June to early September during the major Ethiopian rainy season. Winter recharge is lost through high evaporation–evapotranspiration within the unsaturated zone after relatively long dry periods of high accumulated soil moisture deficits. Chloride mass balance coupled with the isotope results demonstrates the presence of both preferential and piston flow groundwater recharge mechanisms. The stable and radioactive isotope measurements further revealed that groundwater in the Akaki catchment is found to be compartmentalized into zones. Groundwater mixing following the flow paths and topography is complicated by the lithologic complexity. An uncommon, highly depleted stable isotope and zero‐³H groundwater, observed in a nearly east–west stretch through the central sector of the catchment, is coincident with the Filwoha Fault zone. Here, deep circulating meteoric water has lost its isotopic content through exchange reactions with CO₂ originating at deeper sources or it has been recharged with precipitation from a different rainfall regime with a depleted isotopic content. Copyright © 2006 John Wiley & Sons, Ltd.     

Field verification of stable perched groundwater in layered bedrock uplands

Data substantiating perched conditions in layered bedrock uplands are rare and have not been widely reported. Field observations in layered sedimentary bedrock in southwestern Wisconsin, USA, provide evidence of a stable, laterally extensive perched aquifer. Data from a densely instrumented field site show a perched aquifer in shallow dolomite, underlain by a shale-and-dolomite aquitard approximately 25 m thick, which is in turn underlain by sandstone containing a 30-m-thick unsaturated zone above a regional aquifer. Heads in water supply wells indicate that perched conditions extend at least several kilometers into hillsides, which is consistent with published modeling studies. Observations of unsaturated conditions in the sandstone over a 4-year period, historical development of the perched aquifer, and perennial flow from upland springs emanating from the shallow dolomite suggest that perched groundwater is a stable hydrogeologic feature under current climate conditions. Water-table hydrographs exhibit apparent differences in the amount and timing of recharge to the perched and regional flow systems; steep hydraulic gradients and tritium and chloride concentrations suggest there is limited hydraulic connection between the two. Recognition and characterization of perched flow systems have practical importance because their groundwater flow and transport pathways may differ significantly from those in underlying flow systems. Construction of multi-aquifer wells and groundwater withdrawal in perched systems can further alter such pathways.     

Using chlorofluorocarbons (CFCs) and tritium to improve conceptual model of groundwater flow in the South Coast Aquifers of Laizhou Bay,China

The southern coastal plain of Laizhou Bay, which is the area most seriously affected by salt water intrusion in north China, is a large alluvial depression, which represents one of the most important hydrogeological units in the coastal region of northern China. Chlorofluorocarbons (CFCs, including CFC‐11, CFC‐12 and CFC‐113) and tritium were used together for dating groundwater up to 50 years old in the study area. There are two cones of depression, caused by intensive over‐exploitation of fresh groundwater in the south and brine water in the north. The assigned CFC apparent ages for shallow groundwater range from 8 a to >50 a. A binary mixing model based on CFC‐113 and CFC‐12 concentrations in groundwater was used to estimate fractions of young and pre‐modern water in shallow aquifers and to identify groundwater mixing processes during saltwater intrusion. Discordance between concentrations of different CFC compounds indicate that shallow groundwater around the Changyi cone of depression is vulnerable to contamination. Pumping activities, CFC contamination, mixing and/or a large unsaturated zone thickness (e.g. >20 m) may be reasons for some groundwater containing CFCs without tritium. Saline intrusion mainly occurs because of large head gradients between fresh groundwater in the south and saline water bodies in the north, forming a wedge of saline water below/within fresh aquifer layers. Both CFC and tritium dates indicate that the majority of the saline water is from >50 a, with little or no modern seawater component. Based on the distribution of CFC apparent ages, tritium contents plus chemical and physical data, a conceptual model of groundwater flow along the investigated Changyi‐Xiaying transect has been developed to describe the hydrogeological processes. Three regimes are identified from south to north: (i) fresh groundwater zone, with a mixing fraction of 0.80–0.65 ‘young’ water calculated with the CFC binary mixing model (groundwater ages <34 a) and 1.9–7.8TU of tritium; (ii) mixing zone characterized by a mixing fraction of 0.05–0.65 young groundwater (ages of 23–44 a), accompanied by local vertical recharge and upward leakage of older groundwater; and (iii) salt water zone, mostly comprising waters with ages beyond the dating range of both CFCs and tritium. Some shallow groundwater in the north of the Changyi groundwater depression belongs to the >50a water group (iii), indicating slow velocity of groundwater circulation and possible drawing in of saline or deep groundwater that is tracer‐free. Copyright © 2012 John Wiley & Sons, Ltd.     

Assessment of recharge to aquifers / Evaluation de recharge d'aquifères

Abstract

This paper describes a research project based in Cyprus to investigate a number of different methods for determining quantitative recharge to aquifers in semiarid areas. Large lysimeters and geochemical profile techniques have been used in the southeastern Mesaoria area of Cyprus where there has already been a quantitative water resources investigation and where a groundwater model is in preparation. Further joint geochemical and lysimeter studies are also being undertaken in the Akrotiri peninsula (average annual rainfall 430 mm). Excavation and piling techniques have been used to install the lysimeters which are up to 100 m² in area by 4 m deep. The lysimeters have been made as large as possible to minimize edge effects whilst maintaining the soil and aquifer material within them in an ‘undisturbed’ state. A dry drilling method has been developed to obtain sand samples from the unsaturated zone together with a method for processing the core material to obtain data on moisture and solutes. Chloride profiles have been interpreted to provide estimates of the direct recharge component using a steady state, mass balance approach and initial results compare favourably with tritium estimates. The preliminary results for recharge obtained by the different techniques are compared with each other and with estimates obtained from conventional methods. Initial results from the lysimeters indicate a current annual recharge of 5 mm in southeastern Mesaoria (1978), whilst a mean annual recharge of 50 mm at Akrotiri has been obtained by the chloride profile method.     

The environmental tritium concentration of underground water and its hydrological interpretation

Two models which attempt to correlate the tritium concentration of water taken from aquifers to aquifer parameters are discussed. The first takes into account flow along individual streamlines and relates aquifer parameters to the observed tritium concentration at outflow. For the Gambier Plain unconfined aquifer in southern Australia, the calculated tritium concentration at outflow derived from known aquifer parameters and an environmental tritium input function for the area, is 0.7 T.U. The mean tritium concentration of several springs at outflow is also 0.7 T.U.Using the complete mixing model and the approximation that samples withdrawn from an aquifer on Eyre Peninsula, South Australia, are fully mixed, mean annual recharge for the area is estimated at 3 cm/year.     

Hydrochemical Relation Between Shallow Groundwater with Elevated Fluoride and Groundwater in Underlying Carbonates

In the arid to semi-arid district of Chengcheng, Weinan City, in central Shaanxi Province, diminishing groundwater reserves in the shallow Quaternary (QLB) aquifer and elevated fluoride in the similarly shallow Permo-Triassic (PTF) aquifer, have promoted interest in the development of groundwater resources in the deep but poorly understood Cambrian-Ordovician carbonate aquifer system (COC). To investigate the origin of the COC groundwaters and the relationship between the deep and shallower systems, a hydrochemical study was undertaken involving 179 major and minor ion analyses, 39 stable isotope analyses (δD and δ¹⁸O), and 14 carbon isotope analyses (¹⁴C and δ¹³C). PHREEQC 3.0 was used to investigate mixing. Hydrochemical data support the presence of a well-connected regional flow system extending southwards from the more mountainous north. Stable isotope data indicate that the COC groundwaters originate as soil zone infiltration, under a much cooler regime than is found locally today. This is confirmed by ¹⁴C, which indicates the groundwater to be palaeowater recharged during the late Pleistocene (∼10–12 ka B.P.). The presence of nitrate in the COC groundwaters suggests leakage from overlying shallow aquifers currently provides an additional source of COC recharge, with major faults possibly providing the primary pathways for downward vertical flow.     

Water flow and solute transport model of potentially toxic elements through unsaturated zone at regional wellfield Kosnica

The objective of this work was to build a prognostic water flow model and potentially toxic elements (lead, cadmium, zinc) transport model in the unsaturated zone. Research was conducted in the catchment area of Kosnica regional wellfield, where the unsaturated zone is characterised by Fluvisol. Lower sorption capacities were determined in the first horizons for all three potentially toxic elements. Correlation coefficient of the measured and simulated values of tracer concentration is 0.58 for the AC horizon and 0.84 for the 2C/C1 horizon. Based on calibrated water flow and transport parameters, a prognostic water flow model and potentially toxic elements (lead, cadmium, zinc) transport model in the unsaturated zone was built. In case of an accidental spill of potentially toxic elements with concentrations of 1000 mg/l, the risk of contamination of the aquifer is present. Copyright © 2016 John Wiley & Sons, Ltd.     

Transmission of pressure head through the zone of tension saturation in the Lisse effect phenomenon

ABSTRACT

The problem of transmission of pressure head through the zone of tension saturation in the Lisse effect (LE), i.e., the rapid response of groundwater level to pressurized pore air in the unsaturated zone, is investigated theoretically and experimentally. From the law of conservation of energy and the continuity equation, a one-dimensional diffusion equation is derived for transmission of pressure head through the zone of tension saturation. The solution to the equation is the pressure head at any point below the upper boundary of the zone of tension saturation and at any time after the compressed pore air pressure is imposed on the boundary. The key parameter, which determines the behaviour of transmission of pressure head, is the newly proposed pressure head diffusivity coefficient. The theoretical results agree with the experimental results, obtained from laboratory column experiments in three physically different soils.

Editor Demetris Koutsoyiannis Associate editor Xi Chen     

Unsaturated Zone Flow Processes and Aquifer Response Time in the Chalk Aquifer,Brighton, South East England

The Chalk aquifer is one of the main sources of water in South East England. The unsaturated zone in the aquifer plays an important role controlling the time and magnitude of recharge and is major pathway for contaminant transport to the water table. A range of previous work has addressed flow processes in the Chalk unsaturated zone, but physical understanding is still incomplete. Here we present the results of a study on flow mechanism in the Chalk unsaturated zone using a combination of statistical analysis and novel laboratory methods. The study was undertaken at three sites (North Heath Barn [NHB], Pyecombe East [PE], and Preston Park [PP]) on the Chalk of the Brighton block, South East England. Daily and hourly time series data of groundwater level and rainfall were correlated. The results show that a slower groundwater level response to rainfall occurs during dry seasons (summer and autumn) when the amount of effective rainfall is less than 4 mm/day, with a thicker and drier unsaturated zone. A faster response occurs during wet seasons (winter and spring) when the daily effective rainfall exceeds 4 mm/day with a thinner and wetter unsaturated zone. Periods of very rapid response (within 15 h) were observed during wet seasons at NHB and PE sites, with unsaturated hydraulic conductivity (K_u) inferred to reach 839 mm/day. A slower response was observed at an urbanized site (PP) as a result of reduction in direct recharge due to reduced infiltration, due to presences of impermeable infrastructure covering the area around PP borehole. Laboratory measurements of K_u of the Chalk matrix using a geotechnical centrifuge show variation from 4.27 to 0.07 mm/day, according to the level of saturation. Thus, the rapid responses cannot be linked to matrix flow only but indicate the contribution of fracture and karstic flow processes in conducting water.     

Hydrogeologically controlled freshwater dynamics in the coastal sand dune aquifer of Ongole coast (AP), India

Abstract

A sand dune area, ~50 km² in size, the only source of freshwater in the coastal zone of Prakasham district, Andhra Pradesh, India, is bounded by marine sediments in the northwest, and the Bay of Bengal in the southeast. Measurements of groundwater level, hydrochemistry and stable isotopes for three years facilitated the identification of the aquifer response to drought and intense cyclonic storms. There was no major change in hydrochemistry and isotope values between drought and highly saturated conditions, except in a few wells in the northwest. During drought, the groundwater remained fresh, although the levels dipped to 2–5 m b.m.s.l., signifying no saline water ingression (no measurable bromide). Based on the field observations, resistivity soundings, electrical conductivity and groundwater level change due to pumping, the existence of impermeable boundaries in the northwest and southeast are hypothesized. Thus, the existing hydrogeological settings appear to be inhibiting the movement of the freshwater–saline water interface into the freshwater zone.

Editor D. Koutsoyiannis     

Evaluation of an over-used coastal aquifer (Orissa,India) using statistical approaches

Abstract

This study evaluates an over-exploited aquifer (Balasor, India) and also explores the possibilities of sustainable management using several statistical approaches. First, bootstrap analysis indicates that groundwater pumping has resulted in the reduction of mean cultivated area as the average irrigation capacity per bore well dropped from 3.74 ha to 1.5 ha within a period of 10 years of operation. However, modelling the groundwater levels using the seasonal autoregressive integrated moving average (SARIMA) procedure showed no evidence of large-scale groundwater withdrawals. The derived models can be used for water table forecasting and also for infilling the missing observations. The empirical relationship among pumping test results indicated that well depth and aquifer thickness significantly influence the discharge from the aquifer. This discharge may have encouraged the resource-rich farmers to exploit the lower aquifer. Based on a contour plot, the zone of groundwater exploitation was estimated to vary from 25 to 60 m below the surface. Therefore, a uniform aquifer exploitation policy needs to be implemented to curb the vertical competition in exploiting the aquifer and to develop sustainable management of the groundwater.

Citation Panda, D. K. & Kumar, A. (2011) Evaluation of an over-used coastal aquifer (Orissa, India) using statistical approaches. Hydrol. Sci. J. 56(3), 486–497.     

Mapping Coastal Aquifers by Joint Inversion of DC and TEM Soundings-Three Case Histories

Electrical and electromagnetic methods are well suited for coastal aquifer studies because of the large contrast in resistivity between fresh water-bearing and salt water-bearing formations. Interpretation models for these aquifers typically contain four layers: a highly resistive unsaturated zone; a surficial fresh water aquifer of intermediate resistivity; an underlying conductive, salt water saturated aquifer; and resistive substratum. Additional layers may be added to allow for variations in lithology within the fresh water and salt water layers. Two methods are evaluated: direct current resistivity and time domain electromagnetic soundings. Use of each method alone produces nonunique solutions for resistivities and/or thicknesses of the different layers. We show that joint inversion of vertical electric and time domain electromagnetic soundings produces a more tightly constrained interpretation model at three test sites than is produced by inversion methods applied to each data set independently.     

On the leakage assumption applied to equations of groundwater flow

The behaviour and the delayed response of flow to a well in an unconfined aquifer is considered on the basis of recognition of the variable vertical movement of flow inherent in unconfined flow. The effect of elastic storage is taken into account.The typical S-shaped form of the characteristic time-drawdown curve of an unconfined aquifer is analysed. A procedure for determining the unconfined parameters is described.It is shown that the vertical components of flow are a significant factor in the delayed response of an unconfined aquifer while the contribution of the unsaturated zone is essentially subordinate. An equation including allowance for the drainage from the unsaturated zone is derived.     

Estimating groundwater recharge from water isotope (δ2H, δ18O) depth profiles in the Densu River basin,Ghana

Abstract

Accurate estimation of groundwater recharge is essential for the proper management of aquifers. A study of water isotope (δ²H, δ¹⁸O) depth profiles was carried out to estimate groundwater recharge in the Densu River basin in Ghana, at three chosen observation sites that differ in their altitude, geology, climate and vegetation. Water isotopes and water contents were analysed with depth to determine water flow in the unsaturated zone. The measured data showed isotope enrichment in the pore water near the soil surface due to evaporation. Seasonal variations in the isotope signal of the pore water were also observed to a depth of 2.75 m. Below that depth, the seasonal variation of the isotope signal was attenuated due to diffusion/dispersion and low water flow velocities. Groundwater recharge rates were determined by numerical modelling of the unsaturated water flow and water isotope transport. Different groundwater recharge rates were computed at the three observation sites and were found to vary between 94 and 182 mm/year (± max. 7%). Further, the approximate peak-shift method was applied to give information about groundwater recharge rates. Although this simple method neglects variations in flow conditions and only considers advective transport, it yielded mean groundwater recharge rates of 110–250 mm/year (± max. 30%), which were in the same order of magnitude as computed numerical modelling values. Integrating these site-specific groundwater recharge rates to the whole catchment indicates that more water is potentially renewed than consumed nowadays. With increases in population and irrigation, more clean water is required, and knowledge about groundwater recharge rates – essential for improving the groundwater management in the Densu River basin – can be easily obtained by measuring water isotope depth profiles and applying a simple peak-shift approach.

Citation Adomako, D., Maloszewski, P., Stumpp, C., Osae, S. & Akiti, T. T. (2010) Estimating groundwater recharge from water isotope (δ²H, δ¹⁸O) depth profiles in the Densu River basin, Ghana. Hydrol. Sci. J. 55(8), 1405–1416.