Introduction

Water Resources Management (WRM) in karst regions has long been discussed in the literature, due to environmental hazards such as the flood proneness and the sensitivity to pollution (Kemmerly 1981; Mijatovic 1988; Bonacci 2004; Zhou 2007). Moreover, the climate change vulnerability as well as the aquifer recharge issues are nowadays considered main topics for karst environments (Butscher and Huggenberger 2009; Katz et al. 2009; Daher et al. 2011; Ekmekci and Tezcan 2011).

Managed Aquifer Recharge (MAR) deploying treated wastewater, even mixed with rainwater, is a widespread and accepted practice, aiming at increasing groundwater resources (Levine and Asano 2004; Bixio et al. 2008; Drewes 2009; Katz et al. 2009; Scanlon et al. 2016). MAR is frequently implemented in arid and semi-arid zones where saltwater intrusion and soil desertification may occur, and is now considered essential for proper WRM (Georgopoulou et al. 2001; Gale 2005; Daher et al. 2011). Environmental and health problems have been experienced in the many countries where MAR with treated wastewater has been carried out for decades, orienting management authorities toward the release of specific guidelines (NRMMC-EPHC-AHMC 2008; USEPA 2012; EC-JRC 2014). However, hydrological problems also frequently affect the MAR systems. Clogging of infiltration wells and trenches in vadose zones (Bouwer 2002) is an issue of major concern as it can lead to undesirable consequences (Pérez-Paracio and Carrera 1999; Arnon et al. 2005; Song and Liu 2013; Yoo et al. 2013; Chen et al. 2014). The reduction of the permeability of either the filters of the underground structures or the rock pores and fractures may cause the malfunctioning of the surface drainage networks in addition to the reduction of the recharge rate.

Clogging phenomena are the result of interaction between chemical, physical, mechanical, and biological processes (Martin 2013). Quantitative models are available for the prediction of the evolution of clogging in porous media (Blazejewski and Murat-Blazejewska 1997; Pérez-Paracio and Carrera 1999; Mays and Hunt 2005; Skolasinska 2006). Fewer contributions concern fractured rocks (Chrysikopoulos and Abdel-Salam 1997; Ross et al. 2001; Chrysikopoulos et al. 2010) and they are not resolutive because the mechanisms governing the adhesion of organic and inorganic particles to fractured walls are still unclear (Arnon et al. 2005). The quantitative prediction is currently impracticable, in consideration of the difficulty in providing adequate geometrical models of the fracture network. In fractured rocks and especially in karst systems, few discrete features may control the major part of the infiltration in the vadose zone. Consequently, water circulation may be limited even if relatively few fractures and conduits are clogged, specifically those of the near-field networks (epikarst) around infiltration structures as well as natural swallow holesFootnote 1 that commonly constitute the final receptors of endorheic basins (Mijatovic 1988; Hart 2006; Chrysikopoulos et al. 2010).

In the seasonally-arid Salento Peninsula (Lecce Province, Apulia Region, Southern Italy), the deep karstic-limestone aquifer, which provides 75% of the local water supply, is subjected to saltwater intrusion. Thus, the water available for domestic and agricultural uses has a relatively high salt content (Tadolini and Tulipano 1981) and MAR systems may be used to counteract the salinization of the groundwater. However, the swallow holes of these systems may malfunction from clogging. Thus, WRM problems arise, like for the case of the endhoreic Asso Torrent basin, south-west in the peninsula, where the main receptor, a swallow hole named Vora Colucce, provides a reduced discharge capacity. As in other karst catchments (Currens and Graham 1993; Bayless et al. 1995; Milanovic 2002), this main receptor can be utilized for aquifer recharge, as well as runoff regulation and urban defense from intense precipitation (Delle Rose and Fidelibus 2016). In such situations, clogging may lead to increases number and magnitude of the flood events.

The case of Vora Colucce and the Asso Torrent basin is documented because it is representative of the concerns occurring in contexts having hydrological analogies specifically with reference to swallow holes used as receptor of MAR systems. The effect of insufficient wastewater treatment on clogging and the flood hazard are hypothesized to evaluate WRM problems in karst basins. Other issues, such as MAR efficiency, saltwater intrusion, and flood hazards, are also considered because of their relationships with WRM. Examining these site-specific issues, insights are gained that might help avoid unpleasant environmental consequences in similar hydro-geomorphological contexts.

In what follows, the case study and the used methods are shown. In detail, the geological and the hydrological features of the catchment basin and the hydro-geomorphological features of Vora Colucce are reported. The hydrologic model for the estimation of the runoff in the basin and the theoretical evaluation of the treated wastewater quality to deduce the potential amount of clogging material are described. This preliminary study addresses the possible causes of the hydrological disorder in the basin and the plans perspective to improve the WRM.

Case study and methods

The Asso Torrent basin

The Asso Torrent is a dendritic network of natural and artificial channels, crossing several towns and agricultural areas located in the Lecce province of the Salento Peninsula (Fig. 1), and flowing toward six swallow holes, the greatest of them is Vora Colucce.Footnote 2

Fig. 1
figure 1

Location of the Asso Torrent basin

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These receptors are connected, through the vadose zone, to the regional deep karstic-limestone aquifer (Delle Rose and Fidelibus 2016). Due to the nature of karst, chemical and biological processes on the percolating water are limited and the deep aquifer is highly vulnerable to pollution. Salento is a warm temperate area with drought-prone summers (Csa climate type described by Peel et al. 2007—temperate dry-hot summer), mild-wet winters and about 650 mm of average annual rainfall (Zito et al. 1991; Martano et al. 2015). The area is responsive to climate variability, that can affect water resources, ecosystems, human activities, and health (Giorgi and Lionello 2008). Moreover, despite the moderate precipitation reported by historical records, most of Salento region has been recently classified as particularly prone to drought and desertification (Ladisa et al. 2012).

The catchment of the Asso Torrent lies within a graben (i.e., a large tectonic structure lowered by normal faults). Due to the very flat landscape, the watersheds are not clearly identifiable. The endorheicFootnote 3 basin approximately extends 220 km2. According to the regional basin authority (AdBP, Autorità di Bacino della Puglia), the basin area is 256 km2, including also the area drained by an artificial sea channel (Fig. 2). Maximum and average elevations are 198 and 68 m, respectively, with an average slope of 2%. The main channel stretches for about 24 km in total.

Fig. 2
figure 2

Scheme of the Asso Torrent endorheic basin: the solid line delimits the basin, the dashed line includes the extension upon control of the basin authority and the dotted lines are the boundaries between upper, middle, and lower areas of the basin (upper at North). The sewage treatment plants are: 1, Neviano; 2, Aradeo; 3, Maglie; 4, Galatone; 5, Copertino

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The hydrographic system of the Asso Torrent basin is divided into three areas: upper, middle, and lower. The first has a clay-marl substrate (carbonate rocks outcrop close to the watershed) and slopes between 10 and 2%; the substrate of the second consists of limestone and calcarenites and slopes of around 2%; the lower has a calcarenitic substrate (partially covered by clay-marls) and slopes less than 2%. The channels of the upper area are natural, while those of the middle area are a series of hydraulic works, probably constructed by the Romans and later enlarged in the Middle Ages. Finally, the channels of the lower area (north of Nardò village, see Fig. 2) are artificial and built during reclamation works in the 1920s. As a whole, the works transformed the hydrographic system into a sort of centralized open-channel infrastructure, having the function to discharge underground the basin runoff.

Since 1991 the Asso Torrent is used for the disposal of treated wastewater from five sewage treatment plants (STPs) releasing a maximum estimated discharge of 0.2 m3/s (Masciopinto et al. 2012), and from a special treatment plant for industrial sewage. The treated wastewaters mixed with runoff recharge the deep aquifer through the set of swallow holes. The aquifer is used for irrigation by wells placed in the neighboring area (Delle Rose et al. 2000; Delle Rose 2007). This recharge is designed to reduce the seawater intrusion into the deep aquifer (Masciopinto et al. 2012); the system essentially conforms to the aquifer storage, transfer and recovery (ASTR) method of MAR (Dillon et al. 2009).

Severe meteorological events leading to floods have been reported recently for the Asso Torrent basin (Delle Rose and Fidelibus 2016). The first flood-defense works in the basin began in the late 1970s and consisted of a drainage channel flowing to the sea (sea channel in Fig. 2), diverting the Asso Torrent by means of a double sluice gate (G1 in Fig. 2, located north of Nardò). For this channel, the Asso Torrent appears as an exorheicFootnote 4 hydrographic system. The channel should drain the strongest runoffs, but it has not provided the flood protection of Nardò village. In fact, since the 1990s, floods of increased severity have occurred (Table 1), with peak discharges of around 30 m3/s with unpleasant consequences (Delle Rose 2007).

Table 1 Main floods since 1991; rain gauge stations of the regional weather service (Protezione Civile della Puglia): C, Collepasso; G, Galatina; N, Nardò (Delle Rose and Fidelibus 2016)
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These events damaged local agricultural activities (Giordano et al. 2010) and roads, especially those close to Vora Colucce (Fig. 3).

Fig. 3
figure 3

The entrance of Vora Colucce; left: in dry condition at the end of a channel cleaning work (July 24, 2006); right: a flood event (December 4, 2008) resulting from an intense precipitation occurred the day before (see Table 1)

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To address the issues, the European Community Flood Directive (2007/60/CE) and Italian laws (Decreto Legislativo 49/2010) require the use of best practices and technologies to identify areas exposed to floods. New flood defenses, consisting of a new channel (Fig. 2) and accessory works (settling tanks and clusters of injection wells), were designed by considering as input datum a maximum discharge of almost 200 m3/s for a return period of 200 years. The channel was partially completed in 2013; it acts as an overflow channel and is connected to a minor swallow hole with a discharge capacity of 2 m3/s, according to estimations of the local land reclaiming bureau (Consorzio di Bonifica Arneo). The channel flow is controlled by means of a sluice gate (G2 in Fig. 2, located east of Nardò). Finally, as further supplementary hydraulic work, a small channel, branching out from a node close to Vora Colucce toward a dispersal tank and a small natural swallow hole, was built in 2014. In spite of these works, as a consequence of a moderate event of rain (6 March, 2015), a flood affected the rural area around the new channel, causing serious damages to the agricultural production.Footnote 5

The Asso Torrent is currently a fully engineered hydrographic system. With the sluice gates G1 and G2 located between the main channel and the sea and between the main channel and the new channel, respectively (Fig. 2), the following options are available to the authorities: protection of the deep aquifer from seawater intrusion; protection of the groundwater quality; and prevention of urban flooding (Delle Rose and Fidelibus 2016). However, the WRM for the Asso Torrent basin remains difficult because the swallow holes (Fig. 2) are almost completely clogged or have very low discharge rates. An efficient WRM requires relating the hydraulic features of the basin to uncertainties of the precipitation for the next decades. For the Mediterranean areas, including Salento Peninsula, different scenarios have been advanced: a reduction of precipitation during summer (Hatzianastassiou et al. 2016) and even during winter (Somot et al. 2008); an increase of precipitation in summer (Palatella et al. 2010); an increase of extreme rain events coupled with a decrease of annual rainfall, as already occurred throughout the second half of the twentieth century (Alpert et al. 2002). Among these forecasts, damaging winter floods followed by summer droughts would be the worst for the the depletion of the local water resources. An efficient management system requires possible countermeasures for these various scenarios.

Geomorphological and hydrological features of Vora Colucce

Hydraulic engineering has modified the natural shape of Vora Colucce, which is a collapse doline connected with a deeper underground set of cavities. To allow the removal of deposited sediments, during the reclamation works accomplished in the 1920s, three openings were bored in the ceiling of the cavities (Fig. 4). The volume of the swallow hole and cavities is about 850 m3. The level of the deep aquifer is about 30 m below Vora Colucce. Speleological explorations show that the cavity is connected to a rather dense network of fractures and karst conduits (the epikarst), whose hydraulic conductivity as a whole drastically decreases with depth in few meters, and to a vertically aligned karst system. The epikarst affects calcarenite bedrock up to 4–5m below the surface, and the upper portion of the limestone basement (Fig. 4).

Fig. 4
figure 4

Geological vertical section (above) and map (below) of Vora Colucce swallow hole. (1) frequency-weighted rose diagram of joint strikes; (2) calcarenite; (3) limestone basement; (4) detritus

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The discharge capacity through the mouth of Vora Colucce was estimated as 10 m3/s during the reclamation works executed in the first half of the twentieth century (Consorzio Bonifica Arneo 2012). The hydraulic functioning of this karst landform is currently conditioned by the effluents from the STPs that can lead to a physical and biological clogging. The Vora Colucce is now partially filled with sediments and only a limited amount of water may infiltrate.

Tectonic fractures in the near-field were sampled by means of measurements on the outcrop and cavities using a geological compass (Fig. 4). The shallow calcarenite shows four sub-vertical joint systems whose medial trends of dip direction measured from N to E are: 0, 45, 90, and 135 degrees, respectively. The limestone basement shows eight sub-vertical joint systems whose medial trends of dip direction are: 5, 35, 50, 80, 95, 125, 140, and 170 degrees, respectively. Another rock discontinuity is constituted by the sub-horizontal bedding planes. Over geological time, both the tectonic- and the sedimentary-discontinuity sets led to the development of a network of conduits and enlarged fractures that form the local epikarst. Despite of the acquisition of fracture data, the actual geometrical features of the epikarst around the swallow hole are unknown and a quantitative model for the clogging prediction is not available. The main element of the vertically aligned karst system of Vora Colucce is a large shaft whose mouth opens at the bottom of the hypogeum (cavity F, see Fig. 4). This shaft was already filled by high-permeable coarse-sized detritus when the first cave explorers reached it in the 1970s.

The first signals of malfunctioning of the swallow hole occurred in the late 1980s. In 1994, as a consequence of the 03 November 1993 flood event (Table 1), the land reclaiming bureau commissioned expert speleologists to clean the caverns. Plastic materials mixed with silty-sand detritus (a few cubic meters) were manually removed from the bottom of the hypogeum, together with tires, gas cylinders and other large objects (Fig. 5). A thin layer of brownish mud, containing stinky decomposing organic matter, covered the clogging deposits. The large objects and the plastic materials were probably dragged from urbanized areas in Vora Colucce during heavy rains or manually deposited. After this first cleaning, a metal grill (see Fig. 3) was placed to avoid the littering of these objects. Since then, obstruction of the hole is due to muds only.

Fig. 5
figure 5

Cleaning works at Vora Colucce; left: temporary storage of the materials removed on 1994 from the interior of the cavity; right on top: the bottom of the deepest cavern (F in Fig. 4) before the start of the 1994 works; right below: the 27th of August, 2008, a clamshell truck removed organic mud from an artificial well (vertical of cavern D, see Fig. 4)

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However, the discharge capacity of Vora Colucce progressively declined so that in 2000 the land reclaiming bureau commissioned the speleologists to perform a second cleaning. Due to the gas developed by decomposition of the organic matter,Footnote 6 the mud of caverns D and F was only partially removed. The capacity of Vora Colucce to discharge the water of the Asso Torrent continued to decline during the 2010s. During the same period, an increase in flood events occurred (Table 1). The last cleaning, performed in 2008, required the use of clamshells, water jet pumps, and bulldozers (Fig. 5). Hundreds of cubic meters of organic brown mud were displaced from the caverns B, C, D, E, and F. Finally, speleologists manually removed the remaining sediments and attempted to wash fractures and conduits by means of hand-held water jets. However, the swallow hole is currently malfunctioning, despite the last cleaning (Fig. 5). Since the last cleaning, sediments have partially re-filled the cavities and the discharge declined again. Now it is about 1 m3/s (empirically estimated by observing the channel flow at the hole entranceFootnote 7) consistently less than the initial value of 10 m3/s of a century ago (Consorzio Bonifica Arneo 2012).

Basin runoff model

The Vora Colucce area currently undergoes frequent and damaging floods. Therefore, it is useful to evaluate the runoff of the basin. A Curve Number (CN) zonation of the basin is reported herein. CNs should encompass the contribution of land-use changes with time (Ferro 2006). The CN parameters CNI, CNII, and CNIII include the local lithology and the characteristics of both land use and agricultural practices. They range from 0 to 100 and are estimated for the analyzed basin, together with height, slope, surface, and length of main shaft (Mishra and Singh 2003). The CNII value for the Asso Basin is equal to 73. In karst endorheic areas, the use of Soil Conservation Service (SCS)-CN soil classification (USDA 1972) for model parameterization is suggested to evaluate the surface flow rate. The SCS-CN method is not applicable (Iacobellis et al. 2015).

Precipitation is calculated in the basin using the VAPI Apulia methodFootnote 8 (Copertino and Fiorentino 1994), that is appropriate for the estimation of rainfall when specific data are sparse. VAPI is based on the definition of the distribution of the Two-Component Extreme Values (TCEVs, Rossi et al. 1984). It assumes that single rainfalls can be expressed as a mixture of two exponential components: the first one describing ordinary events (more frequent and less severe on average) and the second one referring to extraordinary extremes (more severe and less frequent). The method is used in the context of the Apulian territory and recommended by the regional basin authority (Autorità di Bacino della Puglia 2005).

The rainfall intensity–duration–frequency (IDF) curve is estimated by regional analysis of the annual maxima of rainfall intensity performed with the above-mentioned probabilistic model. In particular, the IDF curves with return period of 30, 200, and 500 years are the following:

$${i_{{\text{3}}0}}={\text{64}}.{\text{6}}{t^{ - 0.{\text{799}}}}$$
(1)
$${i_{{\text{2}}00}}={\text{97}}.{\text{6}}{t^{ - 0.{\text{799}}}}$$
(2)
$${i_{{\text{5}}00}}={\text{113}}.{\text{6}}{t^{ - 0.{\text{799}}}}$$
(3)

where i is the rainfall intensity [mm/h] and t is the rainfall duration [h].

The basin is discretized in sub-basins for the computation. The runoff per unit time, RA [m3/s], coincides with the net rainfall intensity, RB [mm/h], multiplied by the sub-basin area. Intensity RB is equal to RC–RD, where RC is the rainfall intensity and RD the infiltration evaluated by considering the effect of the swallow hole through discharge capacity. The runoff volume W [m3] per each sub-basin derives from the integration of RB in space and time, which is further increased by the upstream overflow volume Wup, when existent. For the long-term, a design hyetograph is used, by approximating RC as uniform in space and time. The IDF for a fixed return period is used for the estimation of RC. The infiltration capacity g(t) is estimated by means of the equation of Horton (1940):

$$g\left( t \right)={g_{\text{c}}}+\left( {{g_0} - {g_{\text{c}}}} \right){e^{ - (t/k)}}$$
(4)

where g0 is the potential infiltration capacity at time t0; gc is the asymptotic infiltration rate at infinite; and k is a coefficient. These parameters change with the location and are calibrated through local inspections of soil and land-use, or otherwise can be defined by referring to literature.

Herein, the data reported in a hydrological study of a conterminous basin (Iacobellis et al. 2015) are adopted. The basin is classified by the regional basin authority using groups A, B, C and D (USDA 1972) conforming to the local geology and referring to the infiltration capacity of the bare soil in average soil moisture antecedent conditions. The description of the groups and the relative values of g0, gc, and k are reported in Table 2.

Table 2 SCS groups and relative suggested values of g0, gc, and k
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The Asso basin is divided into four areas as shown in Fig. 6.

Fig. 6
figure 6

Asso Torrent basin soil classification according to the regional basin authority map (cf. Fig. 2 for topographic reference points)

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Evaluation of contaminant loads from the STPs

A preliminary evaluation of the clogging material in Vora Colucce by the effluents of the five STPs of the basin was carried out. The characteristics, in terms of served population (SP) and plant capacity (PC) of the STPs, are given in Table 3.Footnote 9

Table 3 Characteristics of the STPs extracted from the plan for water protection of the Apulia Region (Regione Puglia 2009)
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By the Italian law, these STP effluents should contain a maximum amount of TSS, BOD, COD, TN and TP equal to 25 mg/L, 20 mg/L, 100 mg/L, 15 mg/L, and 2 mg/L, respectively. Unfortunately, the STPs of the Asso Torrent basin are equipped with only primary and secondary treatment trains, thus there is no treatment for the abatement of TN and TP and the maximum amount of TSS, BOD, and COD in the effluents could reach values equal to 35 mg/L, 25 mg/L, and 125 mg/L, respectively. Moreover, four out of five STPs (Table 3) are undersized and during summer, when the population increases for the tourism, the wastewater production reaches maximum values. In these conditions, the effluents deriving from these STPs are rich in suspended solids and nutrients that constitute fertile environment for the formation of a biofilm around solid particles and at fractured walls in Vora Colucce.

The operative conditions of the undersized STPs can be schematized by dividing the influent flow into two parts. The first part, equal to the product of per-capita supplied water d and PC, is treated. The second part, calculated as the product of d and the difference between SP and PC, is directly delivered into the Asso Torrent without any treatment.

Results and discussion

Possible causes of the hydrological disorder

Given the type of sediments removed from the cavities of Vora Colucce during the above-mentioned cleaning works, the occurrence of a combination of physical and biological clogging phenomena is suggested. It is well recognized that bacteria may adhere to clayey particles, or other suspended solids which have a large surface area and high ion exchange capacity (Kang et al. 2013; Herrera et al. 2004; Huysman and Verstraete 1993). Once these suspended solids begin to clog the voids (pores or fractures), the occurrence of nutrients, such as nitrogen or phosphorus compounds, flowing through the medium, could generate optimal conditions for the growth of a biofilm that further closes the voids (Simpson 2008; Li et al. 2016). To this purpose, by considering the data reported in Table 3, i.e., an average d equal to 250 L/(PE day), and TSS, BOD, COD, TN, and TP discharge equal to 90 g/(PE day), 60 g/(PE day), 150 g/(PE day), 15 g/(PE day), and 2 g/(PE day) (Metcalf and Eddy 2003), respectively, the daily amount of each considered compound discharged in the Asso Torrent basin from the five STPs can be evaluated (Table 4).

Table 4 Daily BOD, COD, TSS, TN, and TP maximum amounts discharged in the Asso Torrent basin
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The BOD, COD and TSS, amounts deriving from the treatment trains are approximately calculated by considering the maximum concentrations allowed by the Italian law (Decreto Legislativo 152/2006), whereas the TN and TP amounts are deduced from the consumption due to the metabolism of the activated sludges of the biological treatments. To this regard, 0.05 g of TN and 0.01 g of TP per each gram of BOD removed have been considered (Metcalf and Eddy 2003). Finally, the daily amounts of BOD, COD, TSS, TN, and TP from the untreated aliquot are evaluated as the product of the difference SP-PC by the relative daily per-capita contaminant contributions.

This theoretical approach probably overestimates the contaminant content in the STP effluents and does not consider the self-purifying power of the river. Consequently, an annual monitoring campaign of BOD, COD, TSS, TN, and TP concentrations in Asso Torrent should be carried out to know the exact amount of contaminants daily reaching Vora Colucce. On the other hand, this evaluation is consistent with the clogging-induced reduction of the discharge capacity of Vora Colucce. In fact, in addition to the TSS, which probably settle along the torrent, the amounts of nitrogen and phosphorous discharged in the basin during the summer season could give rise to biological clogging. Microorganisms may adhere to sediments and fracture walls, forming a biofilm constituted by extracellular polymeric substances (EPS) in which they are embedded (Flemming and Wingender 2010). The reproduction of these colonies, and the consequent thickening of the biofilms, depends on the amount of nutrients in the wastewater. This phenomenon is well-known and even exploited for more than a century to remove nutrients and suspended solids in STPs where a porous bed (generally fine sand) keeps solids and allows the fixing of biofilms (Chaudhary et al. 2003). The accumulation of solids and the formation of a biofilm on the surfaces are the mechanisms promoting the progressive clogging of the pores and fractures that must be periodically cleaned (Bihan and Lessard 2000; Canler and Perret 1994), and can cause the biophysical clogging of the bottom shaft of Vora Colucce (Fig. 7). In addition, it is sufficient that few relatively crucial elements in the entire network of the epikarst are clogged to inhibit the fluid flow.

Fig. 7
figure 7

Bio-physical clogging mechanism in Vora Colucce

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Consequently, the clogging of both the bottom shaft and the epikarst leads to the filling of the entire swallow hole, which happened between 1994 and 2008. It is apparent that, to decrease the proneness to clogging of Vora Colucce, the five STPs should undergo to upgrading works for enhancing the removal of nutrients. Indeed, by increasing the capacity of the STPs and adapting them to the Italian legislation, the maximum amount of contaminants, which can lead to clogging phenomena, daily discharged in Asso Torrent basin would be reduced by almost 70% (Table 5).

Table 5 Daily BOD, COD, TSS, TN, and TP maximum amounts discharged in the Asso Torrent basin after the STPs upgrading works
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In addition, to avoid the intake and subsequent removal of the coarse materials (e.g., tires and gas cylinders), shown in Fig. 5, entrance of Vora Colucce should be protected with wide-mesh grates.

The above considerations may be extended to the minor swallow holes of the hydrographic system (Fig. 2).

The combination of a lower number of rainy days per year with a higher amount of precipitation per rainy day (Alpert et al. 2002; Brunetti et al. 2006) resulted in an increase of the floods, the largest causing significant damages (Table 1). It is worth to note that, in spite of the 2013 hydraulic engineerings, the basin authority has recently defined the area between Nardò and Vora Colucce as affected by a high flood probabilityFootnote 10 (Fig. 8).

Fig. 8
figure 8

Flood probability according to the regional basin authority; left: in the whole Asso basin (cf. Fig. 2 for topographic reference points); right: in the area between Vora Colucce and Nardò

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As shown in Fig. 8, the new channel has moved the flooding hazard from the town to the rural area. Moreover, Vora Colucce would be insufficient to drain stormwater even in absence of clogging phenomena. The results, obtained using the runoff model (Table 6) for three different return periods, show flood volumes higher than the discharge capacity of 10 m3/s. In particular, for a rainfall characterized by a return time of 200 years, a peak rate of 160 m3/s can be estimated, one order of magnitude larger than the maximum discharge.

Table 6 Flood volume for different return periods
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On these bases, in absence of clogging phenomena, Vora Colucce maximum discharge corresponds to a rainfall characterized by return time of only 10 years. Consequently, in addition to the STPs upgrading works, some hydraulic works, such as filter ditches and dry wells, should be carried out to reduce the flood hazard even for high return times.

WRM plan perspective

To ensure the balance between water supply and demand, while minimising the impact on the local environment, efficient WRM plans should be compiled. Long-term balance forecast of water supply is affected by large uncertainties concerning population growth, urbanization, and climate change (Hartmann et al. 2014; Roach et al. 2018). These uncertainties hold also for the Salento Peninsula where, as previously reported, about 75% of the local water demand is currently met by exploitation of the deep aquifer that has been subjected to a salinization for several decades. During the same time, the population of the Salento Peninsula has considerably increased. The data of the Italian Institute of Statistics (ISTAT) show a growth of 22.5% in the Lecce Province between 1951 and 1991, remaining stable since then. The total plant capacity of the STPs discharging in the Asso Torrent corresponds to the 72.5% of the served population (Table 3). Upgraded design for STPs discharging into the Asso Torrent must consider the current deficit as well as the forecasts of population growth.

Throughout the second half of the twentieth century, additional infrastructures, designed by assuming the stationarity of hydrologic variables, were executed (Delle Rose and Fidelibus 2016). Now, a change in paradigm is required (Kundzewicz et al. 2008; Milly et al. 2008). By considering the basin hydro-geomorphological features, the construction of decentralized stormwater infrastructures (especially rainwater harvesting systems) may constitute an efficient environmental policy tool to reduce the runoff toward the swallow holes. Moreover, sustainable practices, such as restoration of old rooftop harvesting (abandoned tens of years ago in favor of centralized systems) are suggested. Besides the reduction of the hydrological disorder, these practices would also allow limiting the demand of water supply, thus reducing the deep aquifer exploitation and, finally, the seawater intrusion.

The efficiency of the recharge systems in any coastal aquifer must be evaluated by considering the consequent reduction of the salinization process (Daher et al. 2011; Mekni and Souissi 2016). According to the value of Total Dissolved Solids (TDS), detected in the deep aquifer at − 5 m below the sea level (Fidelibus et al. 2002), it may be stated that the MAR system has produced a containment of the seawater intrusion limited to the surroundings of Vora Colucce. However, the actual efficiency of MAR to favor the increase of groundwater availability should be evaluated at a regional scale. Unfortunately, literature data on this issue are little and source of conflicting interpretations. The case of the karst Floridan aquifer is emblematic, the recovery evaluation ranging from 25 (Bacchus et al. 2015) to 100% (USACOE 2014). Moreover, appropriate assessments of the effectiveness of MAR systems must be carried out with a multidisciplinary approach, considering also the social costs. In fact, at Voce Colucce, the water infiltrated through the swallow hole contains significant amount of enteric viruses, nitrates and ammonia (Delle Rose 2007). More recently, also the occurrence of pharmaceuticals and disinfection by-products has been noticed (Masciopinto et al. 2012). However, the social cost of diseases caused by the use in agriculture of groundwater pumped around Vora Colucce from the deep aquifer has not been assessed yet.

Conclusions

The water management problems in the Asso Torrent basin include: (1) the almost total clogging of drainage receptors, (2) the occurrence of destructive floods, (3) the current high flood probability in rural area, (4) the limited counteraction of the salinization process in the deep aquifer and (5) the potential danger in agricultural uses of groundwater exploited from wells close to the swallow holes. Such issues confirm the necessity to refuse the old water paradigms based on the assumption of stationarity of the hydrologic variables and the building of centralized water infrastructures.

Examining the site-specific issues, some insights are gained that should be considered to avoid unpleasant environmental consequences in similar hydro-geomorphological contexts and especially in karst endorheic basins. For a correct management of the groundwater recharge and safety of the population, when treated wastewater and rainwater are discharged in swallow holes, several features must be considered. Sewage treatment plants must be correctly sized to match the served population and conform to regulatory limits, to the purpose of avoiding contamination of the groundwater reservoir and reducing the physical and biological phenomena occurring in drainage receptors. By adapting the STPs to the Italian Law, a reduction of the clogging phenomena due to biological processes could be obtained. In basins where water runoff is not externally drained, the reduction of clogging processes is even more stringent, given the increasing flood hazard in the zones close to the drainage receptors. In case of modified systems aimed at increasing the drainage toward the swallow holes, particular care must be given to both the discharge capacity and the runoff peaks to minimize the effects of floods.

The uncertainties about climate change, population growth, and urbanization are challenges for water researchers and managers. The design of engineering and hydraulic structures should be such as to exploit heavy rains to increase the groundwater availability. The principle of stationarity in hydrologic variables must be used with caution or abandoned. Additionally, the paradigm of a centralized infrastructure requires an upgrade. For a sustainable and adaptive perspective, the spreading of decentralized infrastructures at pinpointed areas of the endorheic basins might reduce the demand for water supply as well as the total runoff, thus allowing the decrease of the discharge capacity of drainage receptors for limiting the damages caused by floods. However, where favorable hydrogeologic and climate conditions occur, an increase of intense precipitation may constitute a chance to augment groundwater resources. In coastal aquifers in arid and semi-arid environment, an important goal is containment of seawater intrusion. The need for a multidisciplinary approach seems mandatory and must combine water resources’ protection with wastewater handling and flood defence.