Sodium Levels in the Spring Water, Surface and Groundwater in Dalmatia (Southern Croatia)

A great number of water resources used for water supply occasionally have sodium concentrations exceeding 20 mgL-1 (Tuthill & Calabrese, 1975; EPA, 1996). Thus, in Massachusetts more than 50% of water from community collectors has more than 20 mgL-1 Na. In some areas the sodium concentration exceeds 100 mgL-1. In order to obtain data used for bringing national decisions on monitoring sodium in drinking water Tuthill & Calabrese (1975), from the University of Massachusetts in Amherst, analyzed blood pressure in students from two adjacent water intake areas where all environmental conditions, except the sodium concentration in drinking water, were similar. The students in the water intake area with 105 mgL-1 Na had significantly higher blood pressure than students in the area with a 5 mgL-1 Na concentration. This study points to the necessity of carrying out longitudinal investigations on sodium concentration together with an increased number of data on health.


Introduction
A great number of water resources used for water supply occasionally have sodium concentrations exceeding 20 mgL-1 (Tuthill & Calabrese, 1975;EPA, 1996).Thus, in Massachusetts more than 50% of water from community collectors has more than 20 mgL-1 Na.In some areas the sodium concentration exceeds 100 mgL-1.In order to obtain data used for bringing national decisions on monitoring sodium in drinking water Tuthill & Calabrese (1975), from the University of Massachusetts in Amherst, analyzed blood pressure in students from two adjacent water intake areas where all environmental conditions, except the sodium concentration in drinking water, were similar.The students in the water intake area with 105 mgL-1 Na had significantly higher blood pressure than students in the area with a 5 mgL-1 Na concentration.This study points to the necessity of carrying out longitudinal investigations on sodium concentration together with an increased number of data on health.
In view of the influence sodium in blood has upon the increase in blood pressure, the author decided to classify water as hypotensive (< 10 mgL -1 Na), normotensive (from 11 to 20 mgL -1 Na) and hypertensive (>20 mgL -1 Na) according to the same principle of sodium levels in drinking water.
However, there are significant variations in sodium intake among different individuals as well as from one day to another for each individual (Bauber, 1967).In Western Europe and North America the present consumption of sodium chlorides is estimated to be 5-20 gday-1, with an average of approximately 10 gday-1 (i.e.4g of sodium per day) (WHO, 1979).The minimum daily sodium demand is ca 50 mg in average adults (WHO, 1984).The average daily sodium demand for adults varies from 2-5 g (WHO, 1979).The allowed sodium levels from food and water have been estimated for various groups of adults and children (WHO, 1984).
Excessive sodium concentration can be easily detected through taste.In solutions at room temperature the taste threshold for sodium present in salts as sodium chloride and sodium sulfate is ca 130 to 140 mgL -1 .Generally, the taste is bad at a concentration of <200 mgL -1 (regardless whether it is a chloride or sulfate).However, sensitive individuals can detect a bad taste at concentrations varying from 175 to 185 mgL -1 (WHO, 1979).Therefore, there are built-in restrictions for allowed sodium levels in drinking water.The threshold for sodium taste in water can be changed and it depends on bonded anions, the solution temperature and personal habits for salt intake.
Sodium plays an important role in nutrition (Weast, 1983), contributes to electrolytes regulated by kidneys (Howard & Schrier, 1990), maintains water balance in the body, and affects muscle contraction and the production of adrenaline and amino acids.However, high concentrations of sodium can disrupt cell or blood chemistry.The excessive consummation of sodium may significantly contribute to many diseases, especially hypertension (Dahl & Love, 1957;Hoffman, 1988;Rutan et al., 1988).Sodium is also toxic to plants in high concentrations (Driscoll, 1986).Sodium concentrations above 70 mgL -1 are problematic for irrigation if water is absorbed by leaves (Bouwer, 1978).High concentrations of sodium applied to soils can reduce permeability and produce alkali conditions in which vegetation cannot grow.Elevated sodium levels can also cause problems for industrial uses -for example, foaming in steam boilers.
There is no maximum contamination level (MCL) for sodium, but its guidance level is 20 mg L -1 (EPA, 1996).Sodium-restricted diets can often limit one's total intake to less than 1000 mg day -1 .Assuming a water intake of 2 L mg day -1 , drinking water can contribute substantially to one's total sodium intake at concentrations above 100 mg L -1 .For patients on low sodium diets, drinking water may account for more that 60% of total sodium intake.
Considering the importance of sodium for human health the objectives of the investigations were 1) the systematic monitoring of the sodium levels in the spring water, surface and groundwater in Dalmatia and 2) identification of potential causes of the wide variability in this parameter.

Study area
Dalmatia belongs to karst regions with abundant rainfall.However, it is characterized by insufficient quantities of water since water sinks underground through fissures (Štambuk-Giljanović, 1999).
The groundwater in Dalmatia can be classified as fissure water which, according to hydrochemical and hygienic characteristics, resembles surface water.Groundwater becomes turbid and has to be treated before entering the water supply system.It is moderately hard with a fairly quick flow.Karst waters can be generally classified as calcium-hydrocarbonate water and differ according to the content of sulfates, chlorides, magnesium, sodium and dissolved CO 2 (Štambuk-Giljanović, 2006).
The majority of karst water resources have high quantities of chlorides resulting from the seawater intrusion, i.e. due to the direct mixing of seawater with fresh water or with groundwater flows.These water resources, in addition to increased chlorides content, have increased sulfates, sodium and magnesium contents.

Methods
Individual water samples in Dalmatia (29 taken from spring waters, 25 from surface water and six samples from groundwater) were taken monthly at 60 measurement stations (Figure 1) from January 1, 2010, to January 1, 2011.The sulfates concentrations were measured by employing the turbidimetric method, chlorides by the argentometric method and hardness by the complexometric method according to American standard methods (1995).
According to the concentrations of the mentioned quality parameters the following coefficients were computed: K 1 (Larson & Scold, 1958) (the ratio between the chlorides and sulfates sum and the carbonate hardness) and K 2 (the ratio between noncarbonate and carbonate hardness).Sodium was computed from the difference between K 1 -K 2 and carbonate hardness according to the author's formula (Štambuk-Giljanović, 2003(Štambuk-Giljanović, , 2004(Štambuk-Giljanović, , & 2005)).
According to coefficient K 1 values, the water types were classified into three groups (Štambuk-Giljanović, 2006): a. non-corrosive water with K 1 lower than 0.2; b. water with low corrosiveness, K 1 from 0.2 to 0.65; c. very corrosive water with K 1 higher than 0.65.
The annual results of investigations of waters in Dalmatia were statistically analyzed.They were presented as arithmetic means and medians, since the average concentration values expressed by a median are better than those expressed by an arithmetic mean since the distribution of sodium concentrations is asymmetric.Standard deviations and variability coefficients were computed and the maximum and minimum sodium concentrations in the measurement stations were presented.Statistically analyzed data are presented in Tables 1-3.The tables also present the corrosiveness coefficient K 1 for all the measurement stations.

Results
Sodium concentrations in spring water, surface and groundwater in Dalmatia are presented in Tables 1-3 for 2010 at 60 measurement stations.Sodium concentrations expressed by a median are presented in Figures 2, 4 and 6 while the corrosiveness coefficient K 1 is presented in Figures 3, 5 and 7.
In the spring waters in Dalmatia (Table 1) the sodium concentrations expressed by a median (Figure 2) ranged from 1.8 (Ombla and Biba) to 17.6 mgL -1 (Klokun).The highest variations were noted at the Kotarka station (217%), and the lowest at Opsenica Jurjevići (32%).At the Šilovka station the sodium concentration was 21 mgL -1 with a significant variation (152%).That type of water is classified as hypertensive.According to corrosiveness coefficient K 1 (Figure 3) it is very corrosive water.
Increased sodium concentration at that station results from the infiltration of seawater into the Neretva River due to level oscillations.The seawater influence is greatest from the Neretva estuary to Opuzen while it decreases towards Metković.After the Metković Bridge the seawater action is no longer felt.In other words, after the Metković Bridge the riverbed rises by 2-2.5 m which decreases the infiltration of seawater into the riverbed after the bridge.Maximum sodium concentrations in surface water were from 3.4 (Josetin Most) to 2153 mgL -1 at the Neretva-Opuzen station, while the minimum concentrations varied from less than 0.1 (Josetin Most, Cetina-Prančević Dam, Cetina-Trilj, Cetina-Blato on/C, Ljuta, Jadro at the fishpond entrance) to 10.6 mgL -1 (Neretva-Čapljina).The corrosiveness coefficient K 1 varied from 0.1 (Cetina-Vinalić and Ruda estuary) to 3.2 at Mislina so that this water can be classified as very corrosive water.Among all analyzed 25 surface water resources according to K 1 , 84% of them can be classified as noncorrosive water and 165% as very corrosive water; 64% of surface water can be classified as hypotensive, 20% as normotensive and 16% as hypertensive.
In the Dalmatian groundwater (Table 3) the sodium concentrations expressed by a median (Figure 6) varied from 11.1 mgL -1 at Golubinka to 124.3 mgL -1 at Boljkovac.The greatest variations were recorded at the Nereza station (Dubrovnik area), i.e. 180% while the lowest variations were observed at Golubinka (Zadar area), i.e. 77.3%.At the gauging station Boljkovac the sodium concentration expressed by a median was 124.3 mgL -1 while at Nereza it was 72.7 mgL -1 so that it can be classified as hypertensive water.

Discussion
Dalmatia belongs to the karst region with surface and groundwater karst phenomena; the latter are more complex and result from the geological composition of karst.Thus, karst regions are characterized by permeable limestone with fissures caused by faults and folding.Rainfall water is lost from the surface by sinking underground through limestone fissures which expand due to water outflows and thus create a large storage of groundwater.The groundwater, stored in limestone, flows down to sea level; however, its flow is prevented by impermeable layers or rock layers of low permeability from different lithological formations.Dolomites and dolomite limestone are most important in stopping the groundwater outflow.Those rocks act as insulators preventing groundwater circulation.As barriers, they stop the groundwater flow and force it to appear on the surface in the form of concentrated springs; sometimes groundwater flows can act as siphons and appear in the sea as submarine springs.This barrier is often formed by the sea itself which slows down groundwater flows so that water appears as coastal springs.In the most abundant Dalmatian coastal springs groundwater comes to the surface, flowing from distant karst regions and flooded poljes (fields).It then sinks underground along faults in impermeable rocks or through ponors (swallow holes) along the mountains boundaries.
Closed karst poljes in Dalmatia are catchment areas in which the water is collected and then sinks underground, however, in some places it comes to the surface.(Štambuk-Giljanović, 2006).The majority of springs were used as bases for water intakes for city water supplies in Dalmatian coastal areas.Thus, the Bokanjac groundwater is used for the Zadar area water supply, the Kakma and Biba Springs supply Biograd and Benkovac, the Čikola Spring is used for the Drniš water supply, the Jaruga and Jandrić Springs for Šibenik, the Jadro Spring for Split and Trogir, the Klokun Spring for Ploče, the Ombla Spring for Dubrovnik and the Duboka Ljuta Spring for the water supply of Cavtat and Ćilipe.The Mala Ruda Spring is used for the new Sinj water collector while the Prud Spring supplies Pelješac and Korčula.
The above mentioned large springs represent the only sources of water supply from by springs.The intakes for Dalmatian rivers are on the Cetina River near Gata and Zadvarje and on the Zrmanja River; soon there will be a new intake on the Krka River.
The water resources in Dalmatian karst have more bicarbonates than chlorides and sulfates since water frequently comes into contact with limestone and dolomites while layers of sulfate minerals are rare and less soluble and there are no chloride minerals.In many karst water resources there are significant chlorides concentrations which result from seawater either by direct mixing with freshwater or infiltrating into the groundwater flows.In these flows, apart from increased chlorides concentrations, the sodium and magnesium concentrations are also increased.Sulfates and chlorides in some karst water resources, unlike bicarbonates, vary significantly and depend upon groundwater levels, i.e. rainfall quantities.
Sodium concentrations in Dalmatian water resources expressed by a median during 2010, varied from 1.8 mgL -1 in spring water to 502 mgL -1 in surface water.According to a study carried out in Canada (Subramanian & Méranger, 1984) sodium concentrations expressed by a median ranged only from 0.3 to 242 mgL -1 .
According to the author's classification of drinking water as hypotensive (<10 mgL -1 Na), normotensive (from 11 to 20 mgL -1 Na) and hypertensive (>20 mgL -1 Na) by relating sodium concentration projectively with the arterial pressure and knowing that an increased concentration of sodium in blood increases blood pressure, the following can be concluded: among all analyzed 60 water resources in Dalmatia, 64% can be classified as hypotensive, 20% as normotensive and 16% as hypertensive which means that they exceed the recommended concentration of 20 mgL -1 which is characteristic both for hypotensive and normotensive drinking water.
According to the corrosiveness coefficient K 1 out of all analyzed 60 water samples in Dalmatia, 70% can be classified as noncorrosive water (K 1 lower than 0.2), 6.7% as water with low corrosiveness (K 1 from 0.2 to 0.65) and 23.3% can be classified as very corrosive water (K 1 higher than 0.65).
According to Freis (1973) the primary factor connecting sodium and hypertension is the outcell volume which includes the plasma volume.Sodium is important for hypertension since it is the main factor in the formation of out-cell liquid.Excessive salt in food can be processed by a normal kidney without increasing the volume of out-cell liquid.However, if the kidney does not function properly this volume is increased and hypertension intensified even after a moderate sodium intake.Conversely, at that time sodium quantity should be limited in the diet until the out-cell liquid is reduced which will then significantly decrease blood pressure.
People suffering from hypertension should drink normotensive or even hypotensive water.Sodium concentration in drinking water should be longitudinally analyzed together with an increased number of health data.It is of special importance that data gathered in this way will facilitate the systematic monitoring of sodium in drinking water and ensure quick preventive actions for decreasing sodium concentrations, either by reverse osmosis or by substitution of drinking water sources.Sodium should be included in water quality monitoring since its concentrations can be important to doctors prescribing diets for their patients.

Conclusion
During the year 2010, sodium concentrations were monitored in spring water, surface and groundwater in Dalmatia, in order to study their influence upon human health, i.e. arterial hypertension and cardiovascular diseases.
Sodium concentrations expressed by a median in surface water ranged from 1-502 mgL -1 with variations from 40% to 159%.According to corrosiveness coefficient K 1, 84% of surface water can be classified as non-corrosive and 16% as corrosive.Out of 25 samples of analyzed surface water 64% is classified as hypotensive, 20% as normotensive and 16% as hypertensive.
Sodium concentrations expressed by a median in groundwater ranged from 11.1-124.3mgL -1 with a variation from 77.3% to 180%.According to corrosiveness coefficient K 1 all groundwater can be classified as very corrosive water.The analyzed groundwater was normotensive (67%) and hypertensive (33%).Of all the analyzed 60 samples in Dalmatia, during the year 2010, 70% of the water resources can be classified as non-corrosive, 6.7% as water with a low degree of corrosiveness and 23.3% as very corrosive water.
According to the research results obtained on 60 water samples, 64% can be classified as hypotensive, 20% as normotensive and 16% as hypertensive.
Sodium concentrations and/or water corrosiveness determined by chloride should be used to study the health of each group's drinking water with different sodium (and/or chlorides) concentrations.It can also be used to epidemiologically correlate the pathological states such as arterial hypotension or hypertension with sodium intake from water; it should be also relevant to correlate other pathological states, such as stomach erosion due to water corrosiveness caused by chlorides.