River water quality requirements in determining instream flow

With the 183/89 soil conservation law, the protection of animal life in streams, in Italian environmental regulations, has been specified to the point of explicitly introducing the minimum flow needed to be guaranteed after diversion works. The need not to compromise this flow, denominated by the not altogether fortunate term ‘constant’ minimum acceptable flow, is restated also in subsequent laws. The D.L. 12 July, 1993, no. 275, concerns the legislative rearrangement of public water concession, prescribing that concession be granted duly taking into account the minimum acceptable flow to safeguard, wherever defined, the quality and seasonal equilibrium of the water body. More recently, law 36/94, regulating water resources, prescribes that in basins affected by significant withdrawals or conveyances both downstream and beyond the watershed, diversion works should operate so that the flow necessary to river life survival should be guaranteed, together with the safeguarding of the ecosystem equilibrium. It should be noted that the prescription of the minimum acceptable flow has not been accompanied by rules concerning its quantification. Only at the local level have some territorial agencies, such as the autonomous Italian Provinces of Trento and Bolzano, the regions of Piedmont and Lombardy, issued a number of regulations. These prescribed values can be used as references, as indeed reference quantification values can be borrowed from other countries’ regulations. The need to determine criteria suitable for the Italian situation has recently involved the hydraulic scientific community, leading to interesting contributions to this problem. As a logical general criterion the minimum acceptable flow should be determined on the basis of the preservation of aquatic life in the stream. Some methods have been set up to determine the flow necessary to this end in the hydro-biological context. Two groups can be categorised. One group of methods aims at meeting the needs of determined animal species, and fixes the minimum acceptable flow as a percentage of the yearly mean flow or other typical flow or by means of regression equations that relate it to some representative quantities of the stream. This minimum flow must not be less than a tenth of the stream’s yearly flow. The assumption of the above criteria obviously only has sense for streams similar to those for which they have been laid down and for the achievement of the same aim, that usually is the normal life cycle of a particular kind of fauna. Other methods are based on measurements and assessments to be made on the stream under consideration, evaluating optimum flow or those minima where a current the depth and velocity of which are favourable to maintaining specific habitats (microhabitat method). It should be noted that the value of the instream flow determined according to the above-mentioned criteria could still be insufficient to safeguard the characteristics of the water quality. This situation can occur when the water body is affected by the introduction of pollutants, such as sewage or effluent from wastewater treatment plants. In the past the authors proposed action to mitigate such circumstances, to guarantee minimum quality levels compatible with aquatic life, also taking into account the self-purification capacity of the streams. This methodological approach finds strong support in the recently issued provisions for combating water pollution (Legislative decree 11 May, 1999, no. 152), which reorganise and co-ordinate the regulations so far in force into a single body. The present work aims to draw attention to some criteria concerning: a) the determination of minimum acceptable flow in streams affected by urban sewage, and b) the determination of the degree of treatment to be prescribed for sewage entering streams under drought conditions in order to obtain quality parameters to be maintained along the stream.