The present work provides hydrochemical and stable isotope data and their interpretations for 54 springs and 20 wells, monitored from 2002 to 2006, in the Southern Latium region of Central Italy to identify flow paths, recharge areas and hydrochemical processes governing the evolution of groundwater in this region. The hydrogeological conceptual model of the carbonate aquifers of southern Latium was based on environmental isotopic and hydrochemical investigation techniques to characterize and model these aquifer systems with the aim of achieving proper management and protection of these important resources. Most of the spring samples, issuing from Lepini, Ausoni and Aurunci Mts., are characterized as Ca-Mg-HCO 3 water type, however, some samples show a composition of Na-Cl and mixed Ca-Na-HCO 3-Cl waters. Groundwater samples from Pontina Plain are mostly characterized by Na-Cl and Ca-Cl type waters. Geochemical modeling and saturation index computation of the Lepini, Ausoni Aurunci springs and Pontina Plain wells shows an interaction with carbonate rocks. Most of the spring and well water samples were saturated with respect to calcite and dolomite, however all sampled waters were undersaturated with respect to gypsum and halite. The relationship between δ 18O and δ 2H, for spring and well water samples, shows shifts of both the slope and the deuterium excess when compared to the world meteoric (WMWL) and central Italy meteoric (CIMWL) water lines. The deviation of data points from the meteoric lines can be attributed to evaporation both during the falling of the rain and by run-off on the ground surface before infiltration. Most springs and wells have a deuterium excess above 10 ‰ suggesting the precipitation in the groundwater comes from the Mediterranean sector. On the basis of local isotopic gradients, in combination with topographic and geologic criteria, four recharge areas were identified in the Aurunci Mountains. In Pontina Plain, the elevations of the recharging areas suggest that the Lepini carbonate aquifers are feeding them.
Groundwater constitutes about two thirds of the freshwater resources of the world and plays an important role in human life and economic activity [
Groundwater resources in the southern part of the Latium region play an important role in providing water for domestic, industrial, and agricultural uses (i.e. Mazzoccolo spring, in the Ausoni Mts., is an important source for drinking water supply). Until now quantitative approaches have been developed to study these carbonate aquifers. However, no studies have been undertaken to understand the detailed classifications of groundwater flow systems recharge areas and geochemical evolution of groundwater in this region. The distinct isotopic signature of precipitation has been used in hydrological studies in Italy, however, these studies do not address the spatial isotope pattern of spring and well water from the carbonate aquifers of Sothern Latium. In the present work, groundwater in the carbonate aquifers of the Southern Latium region was characterized employing isotopic and hydrochemical data to identify groundwater flow paths, recharge areas and the main processes control the evolution of water. This study was also designed to hydrochemically characterize and model these aquifer systems with the aim of achieving proper management and protection of these important resources.
Lepini, Ausoni and Aurunci are three distinct groups of mountains belonging to the pre-Apennines of Latium and they occupy a well-defined geographic area, called “Volscian mountain range” (
The Lepini Mts. are located in the northern part of Pontina Plain and they are oriented parallel to the Apennine range, adjacent to the Latium coastline. The Lepini calcareous massif, hosts an important karst aquifer and it may be regarded as an isolated hydrogeological unit, nevertheless there is still some uncertainty about its SE margins. The top of mountains are calcareous, the valley are igneous rocks, which are often covered with younger alluvial fan and colluvial deposits. The Pontina Plain, situated in the southwestern part of the Latium Region, is a coastal plain developed along an extensional marine boundary (
Lepini complex, and the second one is a confined aquifer where the water is discharged from the calcareous aquifer of the Lepini massif and flows to the sea. The aquifer lies under the limestones and it is located at a depth of over 100 m, thus all of the wells sampled within the study area, penetrate the unconfined aquifer [
The Ausoni Mts. (
The Aurunci Mts. represent the southeastern part of the Volscian range and are oriented more or less parallel to the Apennine range (
The main spring and well water sampling survey (
except for seven samples in Pontina Plain, collected in 2003-2006. Water temperature, electrical conductivity and pH values were determined in the field using PC 300 Waterproof Hand-held meter. Bicarbonate content was measured by titration with 0.1 N HCl. Water samples were filtered through cellulose filters (0.45 µm), and their major and minor constituents were determined by a Metrohm 761 Compact IC ion chromatograph (replicability ±2%). A Metropes C2-100 column was used to determine cations (Na+, K+, Mg2+, Ca2+, Li+, Sr2+), while a Metropes A Supp 4 - 250 column was used for anions (F–, Cl–, , , Br–, ,). Chemical analyses were carried out at the Geochemical Laboratory of Sapienza, “University of Rome”. The analytical accuracy of these methods ranged from 2% to 5%. Isotope analyses were conducted at the isotope geochemistry laboratory of the University of Parma. 2H/H isotopes analyses were carried out the method of Kendall and [
where R = D/H or 18O/16O [
Hydrochemical and stable isotope data (18O-2H) were employed to identify the factors (i.e. water types, recharge elevations and flow paths) that control the evolution of water systems of southern Latium region.
Major ion concentrations and physico-chemical characteristics of the analyzed spring and well water samples are presented in
These include: mixing of groundwater (including fluids associated with evaporates) and saltwater, carbonate precipitation and/or diagenesis (e.g. dolomitization), ion exchange and silicate (largely clay) diagenesis and redox reactions [
Pontina Plain) reflects on the presence high concentrations of Na2+, Cl–, and Mg2+ in the coastal area due to seawater intrusion. Obviously, the water rock interactions still in progress. When seawater intrusion occurs, the seawater undergoes chemical changes due to the cation exchange reactions. Generally, the hydrochemical composition of fresh groundwater in coastal carbonate aquifers is typically dominated by Ca2+ and ions. Cation exchangers in the aquifer therefore have mostly Ca2+ on the exchange sites. In seawater, Na+ and Cl– are the dominant ions. Consequently, the adsorbed cation on the sediments in contact with seawater should consist dominantly of Na+. The exchange processes are relevant not only in coastal alluvial aquifers, but they also occur in karstic aquifers, where a minor presence of clays or other exchangers is sufficient to allow salinization to activate ion exchange [
where X indicates the soil exchanger site. The equation shows that Na+ is taken up by the exchanger Ca2+, which is consequently released into the water allowing the formation of Ca-Cl water type. The reverse process can also takes place with refreshening of groundwater when fresh groundwater flushes a salt water aquifer [
Lepini springs: The temperature of Lepini springs range from 10˚C to 15˚C. The pH of these springs ranges from 6.9 to 8.1. The Ca-HCO3 type Lepini springs (LPG1) show a total dissolved solids (TDS) content within the range 101 - 375 mg/l. Calcium and bicarbonate is the dominant constituents in spring samples, belong to the group of Ca-HCO3, followed by magnesium, sodium, sulphate and chloride. The electrical conductivity (EC) value of the springs, belong to the group LPG1, varies from 138 to 473 µS/cm. However, the springs belongs to or have a tendency to the group of Na-Cl type show an increase of total dissolved solids (751 - 1267 mg/l) and electrical conductivity (708 - 1540 µS/cm) values (
Ausoni springs: The temperature of springs ranges from 12˚C to 15˚C. The pH of the Ausoni springs ranges from 7.10 to 7.98 indicating alkaline nature of the water. The EC and TDS values of the springs, belong to the group ASG1, range from 315 to 454 µS/cm and 258 to 351 mg/l, respectively. On the contrary, spring samples of group ASG2 and ASG3 show higher EC (738 - 2310 µS/cm) and TDS (522 - 1321 mg/l) values (
Aurunci springs: The temperature of Aurunci springs ranges from 3˚C to 15˚C except for two samples (AR01 and AR08). The high temperatures (31˚C) may be related to Roccamonfina volcanic system, which is quite close to these springs. The total dissolved solids (TDS) content ranges from 254 to 1153 mg/l. Aurunci springs are alkaline (pH: 7.23 - 8.15) with low to medium electrical conductivity. However, few springs with high total dissolved solids (TDS) and electrical conductivity (EC) is attributed to the more time for water to interact with the host rock (
Pontina Plain: The groundwater temperatures ranges between 12˚C and 20˚C and it increases towards the coast. The groundwater of Pontina Plain show alkaline character with pH values ranging from 7.26 to 8.0 corresponding to carbonate system waters.
As mentioned before, different hydrochemical groundwater types prevail in Pontina Plain and thus, TDS and EC values vary too much. The TDS and EC values of groundwater vary from 340 mg/l to 2804 mg/l and 490 µS/cm - 4180 µS/cm (
Geochemical modelling and saturation index computation of the Lepini, Ausoni Aurunci springs and Pontina Plain wells shows an interaction with carbonate rocks. It can be explained by means of the absorbed carbon dioxide coming from the soil, during recharge, which reacts with the carbonate rocks of the aquifer, dissolving calcite and dolomite according to the following main reactions:
Calculated saturation indexes with respect to various mineral phases (i.e. calcite, dolomite, gypsum and halite) of the spring and groundwater samples are shown in
The degree of hydrochemical evolution is determined by series of ionic ratios of interest (Mg/Ca, Na/Cl, SO4/Cl, HCO3/Cl and Ca/Na) and presented in
dence time of groundwater. The relationship between dolomite dissolution and calcite precipitation is thought to increase Mg/Ca ratios along flow paths (
should be approximately one [
The Na/Cl ratios of spring and well water samples range from 0.08 to 2.25 (
Br and Cl are relatively conservative in hydrological systems and the Br/Cl ratios are often associated to study the origin of chloride and the anomalies of salinity. In the study area, Ca-Mg-HCO3 type spring and well water samples show low Br values (<0.01 mg/l), however for Na-Cl type waters Br/Cl ratios similar to normal and modern seawater ratio of 0.0015 to 0.003, respectively [39,40].
The isotopic composition (18O and 2H) and the calculated deuterium excess values of spring and well water samples, recorded in different periods, are presented in
The comparison of δ18O and δ2H values of spring and well water samples with meteoric water lines (
The precipitation of Mediterranean origin should contribute to the recharge of the regional aquifers.
The deviation of data points from the meteoric lines
can be attributed to evaporation both during the falling of the rain and by run-off on the ground surface before infiltration, i.e. the dotted line shows linear regression of all data. A slope of 4 to 6 is attributed to waters with a significant rate of evaporation relative to input [44,45]. The dotted line has a slope of 4.8 and is calculated from a linear regression on all data. Most of the samples from Lepini springs fall above and below the CIWML, however, spring samples LP01 and LP07 plot below the WMWL suggesting that the water has evaporated or has mixed with evaporated water. The plot of δ2H values versus δ18O for spring samples from Ausoni Mts., collected during April 2002, shows a distribution which tends to converge with the MMWL showing more δ18O enrichment at the expense of δ2H. Samples AS14, AS15 and AS16, collected during May 2004, fall above the CIWML. The measured isotope values of the spring samples from Aurunci Mts., are aligned near to and above the CIWML. In Pontina Plain, the isotope compositions of the groundwater samples plot below the CIWML and close to WMWL. However, PP10, PP11 and PP12, samples fall below the WMWL: it may be related to an evaporation process from falling rain or before infiltration through soils and sand surfaces in the recharge areas (
A deuterium excess of 10‰, the average value for global precipitation, is significantly lower than that of Eastern Mediterranean meteoric water line (EMMWL), where a significant part of the vapour mass has originnated in a closed basin. The low d-values of precipitation reflect slow evaporation at its source region due to high humidity, whereas the high d-values reflect fast evaporation at its source region due to low humidity [
The relationship between δ18O and δ2H, for spring and well water samples, shows shifts of both the slope andthe deuterium excess when compared to the global and central Italy meteoric water lines.
The deuterium excess values has been found in the study area with a range of “d” values minimum +7.3‰ in February 2003 and maximum + 23‰ in April 2002 (
A d-parameter value around 10 ‰ indicates that the water has not been significantly evaporated, and plots on the WMWL and close to the LMWL. The spring and well water samples plot below WMWL show low deuterium excess values (<10‰) indicating the groundwater is isotopically different from its original isotopic composition due to evaporation.
The altitude effect has been used to estimate the mean elevation of recharge of spring and well water samples. The mean isotopic gradient with altitude can be determined directly from the samples collected at a series of sites located at different altitudes, as well as from a series of rainwater samples obtained by different pluviometric stations. As no there is no precipitations isotope data were available for precipitation in the area under study, for the identification of mean isotope gradients, we use the isotope (18O-2H) and elevation values of precipitation, obtained from the four sampling pluviometers near study area (Sabaudia, Roccamonfina, Vairano 1 and Vairano 2).
This methodological process is used to confirm the hypothesis of the conceptual hydrogeological model about groundwater circulation. The altitude effect is found by the relation between precipitation isotope values and elevation in meters (h) highlighting a depletion of heavy stable isotopes of about –0.22‰ and –1.55‰ per 100 m elevation for δ18O and δ2H, respectively (
and well water samples comparing the elevation at which the measured isotopic content of the water samples match the isotopic content of precipitation. As a matter of fact, for the calculation of mean recharge elevation of spring and well water samples, we use:
i) an average weight value of δ18O (–5.13) and δ2H (–27.0) [
ii) for higher elevations in Lepini Mts., the isotope value of δ18O (–7.58) and δ2H (–42.8) of LP08 (Santa Serena) sample as end-member ,whose recharge elevation is less than 1200 m asl.
iii) the isotope value of δ18O (–7.03) and δ2H (–38.3) from end member sample AR23 (Mt. Revole), whose recharge elevation topographically does not exceed 1200 m asl, in Aurunci Mts. for the higher elevations. Identified recharge elevations for each spring and well water samples are presented in
The distribution of δ18O values of sampled waters show a gradual decrease with increasing recharge elevation (
The main directions of identified groundwater flow paths are shown in
residence time (Mg/Ca ~ 0.3) and short groundwater flow paths (
Among these samples AS07, AS08 and AS09, located near the coast, show the highest TDS concentrations (~1300 mg/l) suggesting long groundwater flow path, long residence time (Mg/Ca ~ 0.87) and seawater intrusion (Tables 1 and 3). For the samples AS01, AS02, AS03, AS04, AS15, which are located at the north part of Ausoni Mts., the average discharge elevation is about 95 m asl. The mean recharge elevation for these springs varies between 388 and 662 m asl suggesting long groundwater flow paths, but short residence time (Mg/Ca ~ 0.2) and low TDS concentrations ( ~323 mg/l). Based on the isotopic gradients, three different recharge areas occur in the western Aurunci: two recharge areas are located in the north and the other one is in the south part of the massif. The first area in the north feeds the springs AR23,
AR24, AR25 and AR26. For these springs, the mean recharge elevation range between 916 and 1050 m asl and the average TDS content is less than 300 mg/l indicating short groundwater flow paths and short residence time (Mg/Ca ~ 0.04). Considering the mean recharge elevation of these springs and their low mineralization, it can be assumed that they are recharged by a perched aquifer. The second area in the north recharges the springs AR13, AR14, AR15 and AR16. The mean recharge elevations varies from 229 to 545 m asl for these springs. These springs show average TDS concentrations of 347 mg/l. The recharge area occur in the south part recharges the springs AR18, AR19, AR21 and AR22. The discharge elevation of these spring samples occur at lower elevations, while the mean recharge elevation range between 604 and 938 m asl indicating long groundwater flow paths, short residence time (Mg/Ca ~ 0.3) and low TDS (<325 mg/l) concentrations. In the eastern part of Aurunci Mts., the identified recharge area feeds the springs located at lower elevations (AR01 to AR12 and AR20). For these samples the mean recharge elevations vary from 325 to 712 m asl suggesting long groundwater flow paths. The samples AR01, AR08 and AR20 show higher TDS contents (~1100 mg/l) than other sampled springs, which may be related to the enhanced water-rock interaction along the groundwater flow paths (Tables 1 and 3). Calculated mean recharge elevations, identified by the isotopic analysis, for well water samples from Pontina Plain, range between 652 and 1097 m asl, except for the samples PP10, PP11 and PP12 (329 - 575 m asl). The identified mean recharge elevations for these well water samples (PP01 to PP09) are higher than the recharge elevations of Ausoni springs (<650 m asl), which indicates that the aquifer feeding them is the carbonate aquifer of Lepini Mts. The mean recharge elevations correspond to approximately the topographic elevations of the study area. The samples PP02, PP03 and PP06 show high TDS concentrations (1200 mg·L–1) and medium residence times (Mg/Ca ~ 0.56), which indicates enhanced interaction with calcareous and calcareousdolomitic lithologies and/or seawater intrusion. However, samples PP14, PP16, PP18, PP19 and PP20 show the highest TDS concentrations (>2500 mg/l), according to Lepini, Ausoni and Aurunci springs, which is related to seawater intrusion in the coastal area. In Pontina Plain, the mean recharge elevations of some well water samples (PP13 to PP20) were not determined due to the lack of isotope data.
A combined geochemical and isotopic investigation techniques were applied to spring and well waters from the carbonate aquifers of Southern Latium region, Italy, which are the most important groundwater reservoirs in this area, in order to define in detail the hydrogeological conceptual model of these aquifer systems. The geochemical characterization of spring and well water samples lead to the definition of the aquifers, based on both hydrogeochemical facies and TDS concentrations. In fact, the results of chemical composition and identified recharge elevations of springs and wells allowed us to advance realistic assumptions about the type of aquifer rocks, formed mostly of carbonates, the length of groundwater flow paths and groundwater residence times. The geochemical characteristics of the spring and well water samples, identify different types of groundwater evolution consisting of modifications of chemical composition.
The most common hydrofacies for the spring samples in the Lepini, Ausoni and Aurunci Mts., Ca-Mg-HCO3- type, however some samples show a composition of Na-Cl-type. On the contrary, most of the well water samples from Pontina Plain show the characteristics of a typical composition of Ca-Cl and Na-Cl type waters. The identified hydrochemical facies suggest that water types undergo further geochemical evolution through water rock interaction and/or seawater intrusion in coastal area to reach a final stage of evolution represented by the Na-Cl water type.
The results of geochemical modeling suggest that most of the spring and well water samples are saturated with respect to calcite and dolomite, however all sampled waters were undersaturated with respect to gypsum and halite. The high dissolution rate for carbonate formations allows for waters close to saturation with respect to calcite and dolomite and evaporate minerals (gypsum and halite) to remain undersaturated, resulting in continued dissolution along flow paths. The hydrochemical processes controlling the chemistry of spring and well waters were identified using different ionic ratios (Mg/Ca, Na/Cl, SO4/Cl and HCO3/Cl, Ca/Na and Br/Cl). Most of the groundwater samples from Pontina Plain and low elevation Lepini and Ausoni Mts. springs have molar ratios of these ionic constituents nearly close to the seawater ratio indicating the contribution of seawater. The results of geochemical modelling and the selected ionic ratios shows that the chemical weathering along with the dissolution of rock-forming minerals and seawater intrusion, in the coastal area, which have contributed in the modification of the groundwater chemistry.
The evaluation of δ18O and δ2H values of spring and well water samples shows that most of the samples fall to the left of the WMWL suggesting input to local rainfall that derives from weather fronts coming from the Mediterranean Sea. This fact is also confirmed by the high deuterium excess values (above 14‰) of spring samples suggesting the precipitation in the groundwater comes from the Mediterranean sector. Groundwater samples from Pontina Plain show low d-excess values (ranges from 7.5‰ to 12.5‰) indicating that they were affected by evaporation. The relationship between δ18O and δ2H, for spring and well water samples, shows shifts of both the slope and the deuterium excess when compared to the world meteoric (WMWL) and central Italy meteoric (CIMWL) water lines. The deviation of data points from the meteoric lines can be attributed to evaporation both during the falling of the rain and by run-off on the ground surface before infiltration. Stable isotope values (δ18O and δ2H) of spring and groundwater samples and their relationship between meteoric water lines show that Lepini and Aurunci springs and groundwater samples from Pontina Plain follow what appear to be linear trends that suggest varying source elevations for different springs, while Ausoni springs does not seem to show this trend; suggesting a similar recharge elevation for waters discharging at all springs in the Ausoni Mts. system. Based on the isotopic characterization and geographic positions of the various springs, four different recharge areas identified in the Aurunci Mountains. In Pontina Plain, the elevations of the recharging areas, identified by the isotopic analysis, suggest that the Lepini carbonate aquifers are feeding them.
The primary contribution of this paper was the delineation of potential recharge areas and elevations in several mountain blocks in Central Italy using environmental isotopes, which are a common natural tracer, to predict recharge elevation and constrain recharge area. Geochemistry was also used to infer source region and hydrochemical processes control groundwater composition. The results demonstrate the importance of integrating knowledge of environmental isotopic and hydrochemical characterization for the better management of these aquifer systems in the region.
The authors gratefully acknowledge the financial support of the Regional Basins Authority of Latium (Department of Environment of Latium Region).