Water scarcity

Water can seldom be reused directiy. The treatment required depends on the intended second use. Disposal costs of the wastewater must be included in any economic analysis, and additional treatment for reuse may be justified when this expense is included. Costs of reclamation depend on the location, water scarcity, availabiUty of pubHc water suppHes, and the intended reuse. 

Water Scarcity, Water Stress and Drought, Expressions of Change in River Ecosystems. .. 16... 

Table 2 Summary of effects caused by water scarcity...

Functional Alterations in River Ecosystems Associated with Water Scarcity and Temporality... 

Disturbances that increase water scarcity promote the physical uniformity of river systems and the decrease of biological diversity in streams and rivers. The structure and functioning of heavily impacted river systems become mutually and strikingly similar, irrespective of the river s origin and the climate. The more intense and persistent the disturbance, the greater is the resemblance. On the other hand, river organisms use resources most efficiently in spatially heterogeneous chaimels, and under moderate disturbance frequencies, rather than in steady conditions, to which they are not adapted. 

Metal Ecotoxicology in Fluvial Biofilms Potential Influence of Water Scarcity... 

Table 2 Fate and effects of metals in a stream receiving a point-source of metals (upper part of the table) or diffuse input via urban runoff (lower part of the table). Summary of the expected influence of four different hydrological situations base-flow in a rainy period a flood after a rainy period low-flow after a long period of low rainfall (water scarcity) and a flood produced after this drought. Metal concentration (M) metal retention efficiency (measured on the basis of the nutrient spiraling concept) exposure (dose and duration) bioaccumulation (in fluvial biofilms) and metal sensitivity (of biofihns)...

The first case is based on a hypothetical stream receiving a metal-poUuted source (for instance the outlet of a metal factory). In this case, metal concentration is expected to be driven by dilution, being higher under low-flow than under base-flow conditions and minimum during floods. Metal accumulation is expected to be maximum under low-flow conditions and proportional to the duration of this water scarcity situation. Chronic exposure will lead to community adaptation, which is often related to changes in species composition. Metals will therefore be bioaccumulated in fluvial biofihns and transferred to higher trophic levels in the fluvial food web. 

The second case (Table 2) illustrates a hypothetic urban stream receiving both metals and nutrients mainly via urban runoff. Since urban runoff is directly linked with rainfall episodes, metal inputs and exposure will be variable under base-flow during a rainy period and very low under a situation of water scarcity (low rain and low-flow). In this case, a sudden flow increase after strong rains may cause contrasting effects on water metal transport depending on the previous situation. If previous rains have already washed the metals retained in land and sediments, the... 

Overall the results reported in this review indicate that water scarcity might increase metal exposure (due to low dilution), metal uptake (due to higher retention under low flow), and metal toxicity and/or accumulation (depending on the dose and time of exposure), but also might cause opposite effects depending on the source of pollution. In addition, water scarcity will influence nutrient loads and will also modulate the fate and effects of metals. Thus, future studies addressing the role of environmental stress on the effects of toxicants at community scale are key to predict the impact of toxicants in the aquatic ecosystems. 

Such results constitute a warning for the management of water supply systems based on reservoirs. Considering the major role of streamflow on controlling oxygen levels, water scarcity should be viewed as a threat not only for the quantity of resource, but also for its quality and the cost of treatment. All these... 

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