Current research in urban hydrogeology – A review
Highlights
► Urban groundwater is a valuable resource but often not sufficiently protected. ► Heterogeneous patterns of groundwater contamination result from the complex history. ► We lack studies accounting for long-term change in climate and land use. ► Innovative monitoring approaches are required to define cities as emitters. ► Research in urban hydrogeology is needed to sustainably manage our water resources.
Introduction
Urbanisation is an emerging issue with ecological, economic and social implications. Currently half of the world’s and 70% of Europe’s population is living in urban areas. According to the United Nations, by 2050 these numbers are going to rise to 70% and 84%, respectively [1]. In the year 2000 urbanised areas made up 3.7% of Europe’s surface. Between the years 1990 and 2000 the annual land consumption by housing, services and recreation was 50,000 ha which refers to half of the total land consumption (based on Corine land cover 1990 and 2000 for 23 European countries, http://www.eea.europe.eu/). Of course, there are positive aspects of this development such as more efficient use of land resources and more effective public transport and centralised waste treatment, reducing per capita emissions of contaminants [2]. Nevertheless, urban land use leads to enormous pressure on the environment. Aside from drastic changes in the water balance, manifold and often diffuse and poorly regulated emissions have had a negative impact on the quality of air, soil and urban water resources [2]. On one hand, this environmental stress is likely to increase with further urban growth at an unprecedented rate. On the other hand, the stress factors will change as urban areas undergo dramatic changes, like shrinking or large migration as seen in many cities in the former Eastern Bloc.
Urban water usage as well as urban water quantity and quality problems are closely linked to the city’s development [3]. The different states of development can be seen in all major cities of the world [4]. Over history, settlements often relied on groundwater from springs and shallow wells as a reliable source of clean potable water. With industrialisation and an accelerated urbanisation, water demand has increased. Due to the heavy abstraction, groundwater supplies beneath cities have been declining and as a consequence of unregulated waste management, groundwater quality has become more and more degraded [5]. Cities have increasingly become importers of water from remote sources. Overexploitation of groundwater beneath urban areas, declining water levels, the resulting land subsidence and, for coastal cities, salt water intrusion, still are major concerns in many cities of the world [3], [5], [6]. However, over recent decades, in the developed world, abstraction volumes have been reduced and groundwater levels are rising again. Consequently, pumping has to be increasingly employed to prevent flooding of underground structures [7].
Maintaining the quality and quantity of urban water resources is recognised as a very complex task including different spatial and temporal scales. The key to understand the deterioration of urban water resources is the knowledge of the tremendous impact of urbanisation on the entire water balance (Fig. 1). The deterioration of the water balance can develop very differently in contrasting urban areas and even within heterogeneous cities. Often, surface sealing in urban areas leads to an increase of surface runoff and thus to a reduction of water infiltrating into the subsoil. On the other hand, water is imported into the urban areas by water mains and transported after usage within the sewage system. Water can leak from these subsurface infrastructures as artificial groundwater recharge, increasing the net recharge beneath urban areas [8]. Storm water runoff can also be transported in the sewage system as artificial interflow and mix with wastewater in combined sewers. When the water amount exceeds the capacity of the sewage system, this contaminated storm water can discharge into surface waters (combined sewer overflow, CSO). In urban settings streams as major receivers for groundwater as well as for treated and untreated wastewater are often degraded by a multitude of stressors [9]. This degradation is summarised as the “urban stream syndrome” and comprises amongst others the “flashier” hydrograph with shorter lag times to peak flow, changed base flow magnitude and impaired channel morphology. These effects most likely influence the magnitude and quality of groundwater-surface water interactions. In general the manifold interactions of the different urban water compartments are complex in time and space and still leaves many questions open [10], [11], [12], [13], [14].
The disturbance of the natural water balance is closely connected with deteriorating quality, since new pathways for contaminants are introduced. Probably most challenging is the variety of chemicals from human and industrial activities released via different wastewater sources. We live in a “chemical society” with thousands of chemical compounds available in the products of our daily life [15]. Due to the concentrated accumulation, and the transport and treatment of wastewater in urban areas, urban water resources are at particular risk. The waste-water-borne contaminants are often present in waters in low concentrations ranging from pg L−1 to ng L−1 and are therefore termed “micropollutants” [16]. Examples of micropollutants are pharmaceuticals and personal care products (PPCP) and endocrine disrupting chemicals (EDC) [17], [18], [19]. These compounds are now frequently found in wastewater treatment plants and surface water bodies [20], [21], [22], [23], and although in the last few years several research groups have begun to study these chemicals in urban groundwater (e.g., [17], [24], [25], [26], [27], [28], [29]), they are not usually the focus of groundwater investigations.
Despite the fact that urban-source micropollutants are of concern, urban areas are also often associated with industrial activities which potentially introduce macropollutants such as chlorinated solvents, polycyclic aromatic hydrocarbons (PAHs) and gasoline constituents. In addition, agricultural practises within cities and sewer leakages have contaminated urban aquifers with large amounts of nitrate and phosphate which are still of great concern (e.g., [30]).
The present and future tasks concerning the management of urban water resources are not new. The urban population needs a reliable supply of clean drinking water on the one hand, and on the other hand, urban groundwater contamination and wastewater have to be treated and storm water has to be managed. This task has a substantial overlap with the concept of Integrated Urban Water Management (IUWM). In an IUWM approach water supply, drainage and the sewage systems are seen as parts of an integrated physical system [31]. This approach is a logical consequence of the connection of the water compartments in the urban water balance. Nevertheless, groundwater is often not sufficiently integrated into IUWM concepts [32], [33]. This does not mean we only have to manage the negative effects such as land subsidence, infiltration to the sewage system or building damage by high groundwater levels. Urban groundwater is a heritage and deserves protection and sustainable management in the same way as other water resources. Although being affected by urban land use and anthropogenic activity, a growing number of publications show the value and usability of urban groundwater resources as part of water resources management in urban areas (e.g., [34]).
Urban groundwater can be utilised for potable and non-potable water production (e.g., [35]). Managed and cost-effective aquifer recharge and aquifer storage and recovery methods can be used to recycle storm water or treated sewage for non-potable and indirect potable reuse [36], [37]. Bank filtration of surface water provides potable water for cities like Berlin (Germany) [38]. Foster et al. [39] report the extensive and unregulated usage of shallow urban groundwater in many developing cities as a low-cost alternative to the municipal water supply. Even if the groundwater is not used for water production, urban aquifers are a potential storage location for storm water, reducing surface runoff from sealed areas [40]. Finally, urban groundwater is a valuable energy reservoir since subsurface temperatures are often higher below cities. This significant geothermal potential could be exploited by heat pump installations [41], [42].
Within the framework of a complex urban water balance, the management of urban groundwater has to be carried out very carefully on a sound scientific basis. This includes urban water balancing, the description of contaminant input, transport and fate and the integration into holistic modelling approaches. This review focuses on the challenge to integrate the groundwater component into urban water management. More specifically, we will define the information needed to assess urban groundwater quality and quantity. Subsequently, we review the state-of-the-art of literature with a special focus on adapted methodologies for urban water balancing, the assessment of contaminant concentration distributions in time and space and the estimation of contaminant loads and the implementation of holistic modelling approaches. Being an emerging issue in urban areas, we put special attention to the problem of micropollutants such as pharmaceuticals and personal care products (PPCPs). On this basis, we discuss state and future research needs concerning urban hydrogeology.
Section snippets
Special settings in urban hydrogeology
Urban hydrogeology research has to deal with a high complexity of water flow and contaminant transport for different reasons. In many cases, urban areas developed in geologically interesting and often complex environments, this means close to rivers, hills and other features (e.g. salt water springs and hot springs) which make an investigation a challenge. In addition, urban land use comprises a spatial highly heterogeneous pattern of surface sealing and vegetation which affects groundwater
Overview
For assessing urban groundwater quantity and quality, several types of information as well as evaluation approaches are required. The following sets the scene for the major issues being discussed in this review.
Summary and future research needs
This review is focussed on the state of urban groundwater studies in the context of integrated urban water management. The wide range of literature on urban groundwater issues show that we have to consider the role of urban groundwater in relation to the other and more visible urban water compartments. Urban groundwater is a huge receiver as well as a source for water and solutes in urban areas acting on much longer time scales. Moreover, urban groundwater can be a reactor for contaminants
Acknowledgements
We thank our colleagues from the UFZ-integrated project “Micropollutants in water and soil in the urban environment”. We are grateful for the many discussions with Kristin Schirmer (Eawag) and John Molson (Laval University, Quebec, Canada). We also would like to thank the three anonymous reviewers and the editor Andrea Rinaldo for their valuable comments which helped to improve our manuscript.
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