## 1 Introduction  
Water scarcity in many coastal drylands with limited surface water resources has precipitated high groundwater abstraction rates to supply the increasing urban, tourism, industrial, and agriculture demands [1]. This is the case in some coastal areas in Spain where aquifers play a critical role in sustaining the economy and the environment [2; 3; 4]. The combination of global climate forces underlying human pressures threatens the fragile equilibrium required for a sustainable water supply and the good functioning of groundwater-dependent ecosystems (GDE) [5; 6; 7]. In many densely populated and irrigated drylands, external water transfer from other basins [8; 9], and inner production of non-conventional resources such as urban wastewater reuse and seawater and brackish groundwater desalination to cope with water scarcity are being encouraged [10; 11; 12]. When water is used in urban areas for supply and in peri-urban areas for irrigation agriculture, water fluxes from these areas to regional groundwater bodies occur. The mechanisms for urban wastewater leaching are known. Buried urban sanitation infrastructure might not be watertight due to deterioration, and a fraction of wastewater may leach before reaching the urban wastewater treatment facility. Irrigated agriculture also entails an unavoidable loss of water during irrigation that may reach the aquifers. On favorable topographic and hydrogeological conditions, for instance when steep topography and low-permeability bedrock exist, urban and irrigation returns can be identified in water quantity and quality changes of local springs. Although these mechanisms are well known, the groundwater resource in most medium-sized urban and peri-urban areas is unknown because they are typically ungauged settings. Therefore, conceptualization of the hydrogeological functioning of these areas is crucial to understand and evaluate how these water fluxes can be used to alleviate the pressure on many stressed coastal groundwater bodies. However, geological exposure in urban areas is typically low to define aquifer geometry, especially in flat areas where geological exploration and groundwater monitoring is restricted to some geotechnical soundings and pumping wells at most. The opposite happens in towns with steep topography where geological formations and groundwater dynamics are partially observable and evaluations are possible. These evaluations may serve to conceptualize the groundwater resource generated in neighboring towns having similar habits for water consumption, but a flat topography that prevents direct groundwater observations. Both steep topography and hydrogeological exposure are found in Adra town in southern Spain. Adra town is placed in a steep versant over small aquifers that contribute to the Adra River Delta Groundwater Body (ARDGB), which sustains GDEs, and includes a northern peri-urban area devoted to irrigation agriculture in greenhouses [13; 14; 15]. On the basis of existing or compiled datasets, the groundwater resource evaluation involves three general stages [16; 17; 18]: (1) Aquifer geometry and hydraulics definition, (2) conceptualization of the hydrogeological functioning, and (3) evaluation of the water balance components. This paper is aimed at developing the above two first general stages in deep, whilst the third one evaluates the groundwater resource contributed from the northern steep urban sector (NSUS) to the southern flat urban sector (SFUS), which belongs to the ARDGB. For aquifer geometry definition, previous geological information [19] was revisited and near-surface geophysical techniques were applied. Near-surface geophysical techniques have been widely used in groundwater research to acquire basic information on aquifer geometry and some transient hydrogeological features [20; 21; 22; 23; 24; 25]. This paper combines the Multichannel Analysis of Surface Waves (MASW) [26; 27; 28; 29; 30; 31; 32] and Ground Penetrating Radar (GPR) [33] techniques for these purposes.