Effects of wastewater treatment plants on stream nutrient dynamics under water scarcity conditions

Abstract

Despite an array of significant technological advances, effluents from wastewater treatment plants (WWTPs) generate abrupt physical and chemical discontinuities along the fluvial continuum. These discontinuities not only alter the water quality, but also significantly affect fluvial ecosystem structure and function. In particular, WWTP effluents increase nutrient concentrations and introduce toxic substances, including emergent pollutants, to the fluvial ecosystems. There is abundant evidence that these changes affect stream communities. Fewer studies, however, have examined the influence of WWTP inputs on the hydrologic and nutrient availability regimes. Yet, shifts in these regimes can have clear implications for the stream ecosystem function. This knowledge is of critical relevance for stream management as it provides insights on integrative properties of trophic state, energy transfer, and material cycling at the ecosystem level. WWTP effects on fluvial ecosystems are expected to become more exacerbated under water scarcity conditions, which are characteristic of arid and semiarid regions such as the Mediterranean region. Under these conditions, streams receiving point source inputs may turn into islands of permanent flow within a highly intermittent fluvial network. This has implications not only locally at the reach scale but also at the catchment scale. Within this context, both plant operation and fluvial management may be critical to preserve the ecological integrity of these aquatic ecosystems at the same time that a balance is met with societal demand for high quality water resources. This chapter aims to review current knowledge on this topic, with major emphasis on nutrient loads and in-stream nutrient retention, and to discuss its implications within the context of actual and future scenarios of water scarcity conditions.