The water quality of the Vrgorska Matica River

The article presents the results of investigations carried out on the 42 km long Vrgorska Matica River, which flows through the 15 km long Vrgorsko polje (polje = field) which covers an area of 3000 ha, and is at 24 m a.s.l., located in Southern Croatia. It covers the years 1997-2000 after this field had been reclaimed for agricultural use. The purpose of the investigations was to evaluate the influence of the Vrgorska Matica River which is part of the catchment area of the Trebizat River, on the water quality in Modro Oko Lake and Prud Spring, which are used for water supply and are located downstream of the Vrgorska Matica River on the right bank of the Neretva River. The water quality was evaluated by using the quality index based on the following nine parameters: temperature, mineralization, corrosion coefficient, K =(Cl + SO4)/HCO3, dissolved oxygen, BOD5, total N, protein N, total phosphorus and total coliform bacteria (100 mL)(-1) (MPN coli (100 mL)(-1)) for which concentrations C-95 are calculated. After completing the nine parameters the results of C-95 were recorded and transferred to the score table to obtain the q-value. The q-value used is an attempt to quantify environmental factors which would otherwise be qualitative. For each parameter the q-value was multiplied by a weighting factor based upon the relative significance of the parameter. The nine resulting scores values were then added to arrive at an overall water quality index (SigmaS(95)). According to this index the water can be classified into four categories. The first category, according to the Croatian Water Classification Act (Official Bulletin No. 77,1998), includes ground and surface waters used for drinking or in the food industry either in its natural state or after disinfection, and surface water used for raising high-quality species of fish, ranging from 85-100 scores; the second category includes water used in its natural state for swimming and recreation, sports or for other species of fish and the treated water used for drinking and other industrial purposes ranging between 70-85 scores. The third category includes water used in industry with no specific requests upon water quality and in agriculture ranging from 50-70 scores; the fourth category includes water used only after being treated in areas with water shortage which is less than 50 scores. According to results obtained by investigations, the water of the Vrgorska Matica River and Prud Spring falls into the second (II) category, while water from the Modro Oko Lake belongs to the first (I) category. This means that the Matica River water does not influence the water quality of the Modro Oko Lake. This lake water quality is influenced by the Rastocka Matica River from the Rastocko polje which is located upstream from the Vrgorska Matica River. This has been proved by dyeing tests.