Relative influence of changes in hydraulic conductivity with depth and climate change on estimations of borehole yields

Understanding the impact of climate change on borehole yields from fractured aquifers is essential for future management of groundwater resources. Although the impact of changes in hydraulic conductivity with depth (VKD) on groundwater levels is well established, the relative significance of climate change and VKD on borehole yield estimates is poorly understood. We hypothesize that VKD exerts a significant additional control on borehole yields under climate change which has not been considered in yield assessments to date. We developed a radial groundwater flow model of an idealised pumping borehole in the fractured Chalk aquifer of south-east England, and applied 11 VKD profiles based on a simple conceptual representation of variability in hydraulic conductivity with depth in the Chalk. For each VKD profile, we applied 20 climate scenarios and six constant pumping rates for the period 1962 – 2014. We then estimated borehole yields based on the derived lowest pumping water levels during key drought years (e.g. 1976). We show that VKD is more significant (p 0.1) in controlling lowest pumping groundwater levels. Hydraulic conductivity is as significant a control as climate on borehole yields, although responses are highly non-linear associated with pumping water level-pumping rate curves intersecting key yield constraints (e.g. pump intake depth, major inflow horizons). It is recommended that variations in hydraulic conductivity with depth are taken into consideration in future assessments of borehole yields under climate change when developing integrated water resources management plans. The approach presented is generic and can be applied across different aquifers where vertical heterogeneity is present

Isotopic fingerprint for phosphorus in drinking water supplies

Phosphate dosing of drinking water supplies, coupled with leakage from distribution networks, represents a significant input of phosphorus to the environment. The oxygen isotope composition of phosphate (δ18OPO4), a novel stable isotope tracer for phosphorus, offers new opportunities to understand the importance of phosphorus derived from sources such as drinking water. We report the first assessment of δ18OPO4 within drinking water supplies. A total of 40 samples from phosphate-dosed distribution networks were analyzed from across England and Wales. In addition, samples of the source orthophosphoric acid used for dosing were also analyzed. Two distinct isotopic signatures for drinking water were identified (average = +13.2 or +19.7‰), primarily determined by δ18OPO4 of the source acid (average = +12.4 or +19.7‰). Dependent upon the source acid used, drinking water δ18OPO4 appears isotopically distinct from a number of other phosphorus sources. Isotopic offsets from the source acid ranging from −0.9 to +2.8‰ were observed. There was little evidence that equilibrium isotope fractionation dominated within the networks, with offsets from temperature-dependent equilibrium ranging from −4.8 to +4.2‰. While partial equilibrium fractionation may have occurred, kinetic effects associated with microbial uptake of phosphorus or abiotic sorption and dissolution reactions may also contribute to δ18OPO4 within drinking water supplies

Improved understanding of spatio-temporal controls on regional scale groundwater flooding using hydrograph analysis and impulse response functions

Controls on the spatiotemporal extent of groundwater flooding are poorly understood, despite the long duration of groundwater flood events and distinct social and economic impacts. We developed a novel approach using statistical analysis of groundwater level hydrographs and impulse response functions (IRFs) and applied it to the 2013/14 Chalk groundwater flooding in the English Lowlands. We proposed a standardised index of groundwater flooding which we calculated for monthly groundwater levels for 26 boreholes in the Chalk. We grouped these standardised series using k-means cluster analysis and cross-correlated the cluster centroids with the Standardised Precipitation Index (SPI) accumulated over time intervals between 1 and 60 months. This analysis reveals two spatially coherent groups of standardised hydrographs which responded to precipitation over different timescales. We estimated IRF models of the groundwater level response to effective precipitation for three boreholes in each group. The IRF models corroborate the SPI analysis showing different response functions between the groups. We applied identical effective precipitation inputs to each of the IRF models and observed differences between the hydrographs from each group. It is suggested this is due to the hydrogeological properties of the Chalk and of overlying relatively low permeability superficial deposits (recent unconsolidated sediments overlying the bedrock, such as clays and tills), which are extensive over one of the groups. The overarching controls on groundwater flood response are concluded to be a complex combination of antecedent conditions, rainfall and catchment hydrogeological properties. These controls should be taken into consideration when anticipating and managing future groundwater flood events. The approach presented is generic and parsimonious and can be easily applied where sufficient groundwater level and rainfall data are available

Watermains leakage and outdoor water use are responsible for significant phosphorus fluxes to the environment across the United States

Human activity has led to excess phosphorus (P) concentrations and the continued eutrophication of coastal and freshwaters across the United States (US). Developing more effective P management policy requires a comprehensive understanding of P sources in the environment. Public water systems across the United States widely dose water with phosphate (PO4) in order to control the corrosion of lead and copper within their distribution networks. Using public water system PO4 dosing facility data and target PO4-P dosing concentrations, we estimate that PO4 dosing added 4–14.9 kt PO4-P yr−1 into the US water distribution network in 2015. Using estimates of public water supply inputs and domestic water deliveries, we estimate that 0.7–2.6, and 0.8–3.1 kt PO4-P yr−1 were then lost from the network due to watermains leakage and outdoor water use, respectively. After accounting for these fluxes, we estimate that 9.3 kt PO4-P yr−1 was then returned to wastewater treatment plants (WWTPs) and accounted for up to 2.7% of the national WWTP influent P load. As sources of P to the environment, lower and upper estimates of combined watermains leakage and outdoor water use PO4-P fluxes exceeded P loads to surface waterbodies from documented point sources across 461–541 counties. The exceedance of these fluxes above other major components of the US P-budget emphasizes the need to include them in P source apportionment studies, both across the US and in other countries where public water supplies are dosed with PO4

Future changes and uncertainty in decision-relevant measures of East African climate

The need for the development of adaptation strategies for climate change in Africa is becoming critical. For example, infrastructure with a long lifespan now needs to be designed or adapted to account for a future climate that will be different from the past or present. There is a growing necessity for the climate information used in decision making to change from traditional science-driven metrics to decision-driven metrics. This is particularly relevant in East Africa, where limited adaptation and socio-economic capacity make this region acutely vulnerable to climate change. Here, we employ an interdisciplinary consultation process to define and analyse a number of such decision-oriented metrics. These metrics take a holistic approach, addressing the key East African sectors of agriculture, water supply, fisheries, flood management, urban infrastructure and urban health. A multifaceted analysis of multimodel climate projections then provides a repository of user-focused information on climate change and its uncertainties, for all metrics and seasons and two future time horizons. The spatial character and large intermodel uncertainty of changes in temperature and rainfall metrics are described, as well as those of other relevant metrics such as evaporation and solar radiation. Intermodel relationships amongst metrics are also explored, with two clear clusters forming around rainfall and temperature metrics. This latter analysis determines the extent to which model weights could, or could not, be applied across multiple climate metrics. Further work must now focus on maximising the utility of model projections, and developing tailored risk-based communication strategies

The influence of groundwater abstraction on interpreting climate controls and extreme recharge events from well hydrographs in semi-arid South Africa

There is a scarcity of long-term groundwater hydrographs from sub-Saharan Africa to investigate groundwater sustainability, processes and controls. This paper presents an analysis of 21 hydrographs from semi-arid South Africa. Hydrographs from 1980 to 2000 were converted to standardised groundwater level indices and rationalised into four types (C1–C4) using hierarchical cluster analysis. Mean hydrographs for each type were cross-correlated with standardised precipitation and streamflow indices. Relationships with the El Niño–Southern Oscillation (ENSO) were also investigated. The four hydrograph types show a transition of autocorrelation over increasing timescales and increasingly subdued responses to rainfall. Type C1 strongly relates to rainfall, responding in most years, whereas C4 notably responds to only a single extreme event in 2000 and has limited relationship with rainfall. Types C2, C3 and C4 have stronger statistical relationships with standardised streamflow than standardised rainfall. C3 and C4 changes are significantly (p < 0.05) correlated to the mean wet season ENSO anomaly, indicating a tendency for substantial or minimal recharge to occur during extreme negative and positive ENSO years, respectively. The range of different hydrograph types, sometimes within only a few kilometres of each other, appears to be a result of abstraction interference and cannot be confidently attributed to variations in climate or hydrogeological setting. It is possible that high groundwater abstraction near C3/C4 sites masks frequent small-scale recharge events observed at C1/C2 sites, resulting in extreme events associated with negative ENSO years being more visible in the time series

Observed controls on resilience of groundwater to climate variability in sub-Saharan Africa

Groundwater in sub-Saharan Africa supports livelihoods and poverty alleviation1,2, maintains vital ecosystems, and strongly influences terrestrial water and energy budgets. Yet the hydrological processes that govern groundwater recharge and sustainability—and their sensitivity to climatic variability—are poorly constrained4. Given the absence of firm observational constraints, it remains to be seen whether model-based projections of decreased water resources in dry parts of the region4 are justified. Here we show, through analysis of multidecadal groundwater hydrographs across sub-Saharan Africa, that levels of aridity dictate the predominant recharge processes, whereas local hydrogeology influences the type and sensitivity of precipitation–recharge relationships. Recharge in some humid locations varies by as little as five per cent (by coefficient of variation) across a wide range of annual precipitation values. Other regions, by contrast, show roughly linear precipitation–recharge relationships, with precipitation thresholds (of roughly ten millimetres or less per day) governing the initiation of recharge. These thresholds tend to rise as aridity increases, and recharge in drylands is more episodic and increasingly dominated by focused recharge through losses from ephemeral overland flows. Extreme annual recharge is commonly associated with intense rainfall and flooding events, themselves often driven by large-scale climate controls. Intense precipitation, even during years of lower overall precipitation, produces some of the largest years of recharge in some dry subtropical locations. Our results therefore challenge the ‘high certainty’ consensus regarding decreasing water resources in such regions of sub-Saharan Africa. The potential resilience of groundwater to climate variability in many areas that is revealed by these precipitation–recharge relationships is essential for informing reliable predictions of climate-change impacts and adaptation strategies

Analysis of the impact of hydraulic properties and climate change on estimations of borehole yields

Understanding the impact of climate change on borehole yields from fractured aquifers is essential for future water resources planning and management. Although variation in hydraulic conductivity with depth (VKD) in fractured aquifers is a well-known phenomenon, the relative significance of climate change and VKD on borehole yield estimates is poorly understood. We hypothesize that VKD exerts a significant additional control on borehole yields under climate change that has not been considered in yield assessments to date. We developed a simple two-layered radial groundwater flow model of an idealised pumping borehole in the fractured Chalk aquifer of south-east England, and applied 11 VKD profiles based on a simple conceptual representation of variability in hydraulic conductivity with depth in the Chalk. For each time step, the transmissivity is calculated by integrating the hydraulic conductivity VKD profile over the saturated depth calculated at the previous time step. For each VKD and resulting transmissivity, we applied 20 climate scenarios and six constant pumping rates for the period 1962 – 2014. We then estimated borehole yields based on the derived lowest pumping water levels during key drought years (e.g. 1976). We show that the hydraulic properties of the aquifer are more significant (p 0.1) in controlling lowest pumping groundwater levels when abstraction rates are < 9000 m3/day, and that both are significant when abstraction ≥ 9000 m3/day. Hydraulic conductivity is as significant a control as climate on borehole yields, although responses are non-linear associated with whether pumping water level-pumping rate curves intersect key yield constraints (e.g. pump intake depth, major inflow horizons). It is recommended that variations in hydraulic conductivity with depth are taken into consideration in future assessments of borehole yields under climate change. The approach presented is generic and can be applied across different aquifers where vertical heterogeneity is present and affects transmissivity

Water supply processes are responsible for significant nitrogen fluxes across the United States

Excessive nutrient concentrations within fresh waters are a globally persistent problem. Developing effective nutrient management strategies requires improvements to nitrogen (N) mass balances, including the identification and quantification of previously unrecognized anthropogenic N fluxes. Using publicly available data, we establish that freshwater abstractions from both surface waters and groundwaters, alongside watermains leakage from public distribution networks, are responsible for significant nitrate-N (NO3-N) fluxes across the contiguous United States. Nationally, freshwater abstraction temporarily retains 417 (min-max: 190-857) kt NO3-N yr-1, equivalent to 21% of pastureland N uptake and 2% of previous global abstraction-N flux estimates. Fluxes due to irrigation, thermoelectric power and public water supply collectively account for 87% of this total. We find large inter-county variation in area-normalized abstraction fluxes (min-max: 0-8,267 kg NO3-N km-2 yr-1), with eastern regions generally associated with larger fluxes. Watermains leakage returns 7 (min-max: 6.3-7.7) kt NO3-N yr-1 back to the environment, equivalent to 13% of NO3-N initially abstracted for public supply and 1.3% of previous global leakage flux estimates. Our analyses reveal inter-county variations in area-normalized leakage fluxes (min-max: 0-576 kg NO3-N km-2 yr-1), with this flux exceeding other major N inputs (agricultural N fertilizer) in some urbanized and coastal counties, highlighting their importance in these areas. The local and national importance of these fluxes has implications for policy makers and water resource managers aiming to better manage the impacts of N within the environment and calls for their inclusion in both US and global N budgets