## 4 Discussion  
The drainage water of the study area was characterized by its low quality, mainly due to the high presence of salts. This water is commonly reused for irrigation based on an ancient system of water distribution and scarce resources. A severe water deficit in late summer implies that farmers reuse the drainage water to irrigate their crops. It produces salt enrichment, and other compounds, as they continuously recirculate and lixiviate thought the soil (and evaporating water too). According to the irrigation water quality criteria developed by [PERSON] [38], about 85% of our water samples have a severe degree of restriction for use (EC > 3.0 mS/cm). This fact greatly explains the selection and spatial distribution of crops in the study area. For example, in Figure 1, _lower_ citrus crops (CLC2018 level 3, code 222, fruit trees and berry plantations) surround lower salinity drainage channels in the Segura River channels, while tolerant plants, e.g., melon or date palms, are irrigated with higher salinity water, mainly proceeding from the Vinalopo River channels. This spatial pattern is also coherent with the spatial pattern of soil electrical conductivity, which we previously found in the study area [23; 39]. Secondary soil and water salinization is widespread through intensive agriculture irrigated floodplains of southeast Spain, where solute inputs from irrigation and rainfall are much higher than the solute outputs of plant uptake or leaching down the soil [40]. Additionally, the Vinalopo River (EC = 18.2 \(\mu\)S/cm) transports high amounts of soluble salt from upstream Triassic Keuper deposits [41]. It is artificially drained from the flooding area, where the wetlands of the ancient lagoon and marshes were almost completely desicated for agriculture [21]. All of these factors might explain the spatial distribution of water EC values and the location of the high-high cluster of salinized water; and it was found for soils [23; 39]. The presence of wetlands in the study area represent highly valuable environmental and socioeconomic resources. Unfortunately, they were traditional hotspots for malaria, and the employment of DDT and other insecticides was very intense for several decades of the 20 th century [42]. Current employment of DDT should be null, due to the fact that DDT in the EU was banned in 1978 (Directive 79/117/EEC). However, residual DDT presence is widespread and has been repeatedly found in marine sediment, seawater, or even seabirds [43; 44]. We reported a DDT-DDE mean value in the study area of 1.45 \(\mu\)g/L with a standard deviation of 0.87 \(\mu\)g/L and a maximum of 6.53 \(\mu\)g/L. DDT-DDE was detected in 69 of the 76 water samples (90.8%), suggesting that its current presence is ubiquitous through the study area. As a reference of the importance of this pollution, the WHO guidelines for drinking water provide guideline values of DDT and metabolites, of 1 \(\mu\)g/L [45]. In this sense, 69.6% of our water samples (48 out of 69) were above this guideline value. Concerning glyphosate, this herbicide is currently commercialized and its use for weed control is approved in the European Union until 15 December 2022 [46]. We reported an average glyphosate value in the study area of 2.04 \(\mu\)g/L, with a standard deviation of 1.32 \(\mu\)g/L and a maximum of 6.24 \(\mu\)g/L. The WHO provides a guideline value for glyphosate, along with its metabolite aminomethylphosphonic acid (AMPA) [45]. A health-based value of 0.9 mg/L (equivalent to 900 \(\mu\)g/L) can be derived based on the acceptable daily intake (ADI) of 0.3 mg/kg of body weight, assuming a 60-kg adult consuming 2 L of drinking water per day, and allocating 10% of the ADI to drinking water [47]. Not one of our water samples was above this guideline value. Our selection of the ELISA test for the quantification of pesticides in water samples was based on previous evidence about its suitability for efficient and cost-effective monitoring of water pollutants.