River Boyne Water Quality Management Plan - EPA

River Boynr Water Quality Msnagrmmt Plan5.4 Troplric Status ond Associnted Pltosplrate Standards in Boyne River SystemPhosphates (PO4), generally expressed as mg/l P, occur widely in nature and are significantconstituents of detergents, particularly those for domestic use, as well as being widely used as anagricultural fertiliser. Run-off and sewage discharges are therefore important contributors ofphosphates to surface waters.The significance of phosphorous is principally in regard to eutrophication (over-enrichment) of lakesand, to a lesser extent, rivers. In combination with nitrate, phosphate in such water bodies promotesthe growth of algae and other plants leading to blooms, littoral slimes, etc. Daytime photosynthesisand night-time respiration give rise to excessive dissolved oxygen levels in daytime (supersaturation)and depleted levels at night-time, tcgether with other changes such as increased pH.Phosphates can be present in solution, in colloidal suspension or adsorbed onto particulate matter.The element exists in bound and unbound forms which are difficult to separate totally. The termorthophosphate is a widely used measure of bioavailability of this important nutrient.In lake waters, concentrations above 0.01 mgll P are considered likely to promote excessive algalgrowth. Extensive research is available on the determination of appropriate limits for particular lakeconditions. For surface water abstractions, for Class A2 waters,& limit of 0.3 mg/l P is adopted.There are no limits in the Salmonid Regulations, but a fig$ of 0.2 mgll, expressed as PO4,equivalent to 0.0652 mg/l as P, is stated as indicative in,wdpLto reduce 5utrophica:ion. A tigure ofO.lmg/l P has been suggested by the U,$. EPA (~~~'(976)-U' \ for total phospl~orous in rivers andstreams.Phosphorous occurs in both 'available' a,@I$$navailable' forms. Vie 'ava.ilsble' fcnn ofphosphorous, as dissolved inorganic phoq$$$e (orthophosphate), is predominantly taken up byplants and algae for growth. ~hosphatek?~$ sewage effluent and farmyard waste are generally inLmthe available form.&.*Background catchment phosphatgfioad varies considerably, depending on the catchment soilclassification, as indicated on the following Table.Classification of Soils by Phosphate LevelsFor inspection purposes only.Consent of copyright owner required for any other use.Soil TypeCharacteristicTotal Phosphorus Exportmg P/m2-yrOligotrophic SoilsMesotrophic SoilsPolytrophic Soils............... -..........................................................................................................................................Low Nutrient LossesQ0............................................................................ Medium Nutrient Losses ............................................Large Nuhient Losses20 - 50".>50The catchment to Navan Weir (1610km2) was analysed to assess the phosphate export load from theBoyne catchment. Available data indicated the median total phosphate concentration as 0.065mg/l P.The median concentration of ortho-P was calculated as 0.040 mgll P, representing 62% of the total - Pload. This data results in the following phosphate balance at Navan weir.EPA Export 26-07-2013:17:27:01