Dynamics of surface water quality driven by distinct urbanization patterns and storms in a Portuguese peri-urban catchment

Abstract

Although the influences of urban land use on water quality have been widely investigated, the impacts of different urbanization patterns, particularly in Mediterranean environments, are not well understood. Focussing on a Portuguese peri-urban catchment with 40 % urban cover, this paper explores (1) the impact of areas with differing urban extent and storm drainage system on streamwater quality and (2) temporal variations driven by season and storm events of differing magnitude, intensity and antecedent weather. Water quality was assessed at the catchment outlet (E) and for three upstream tributaries: (1) Porto Bordalo (PB), 39 % urban with a new major road and piping of some overland flow from impervious surfaces directly into the stream; (2) Espirito Santo (ES), 49 % urban, mostly comprising detached houses surrounded by gardens, and with overland flow infiltrating into downslope pervious soils; and (3) Quinta (Q), 22 % urban with partial piping of overland flow from a recent enterprise park area. Water samples were collected at different stages in storm hydrograph responses to ten rainfall events from October 2011 to March 2013. The water quality variables analysed included chemical oxygen demand (COD), nutrients (Kjeldahl nitrogen [Nk-N], ammonium [NH4–N], nitrate [NO3–N] and total dissolved phosphorus (TDP) and heavy metals (Zn and Cu). Urban areas had great impact on COD, with the highest median concentrations in ES and the lowest in Q. In ES, fertilizing lawns and gardens may have been responsible for its higher median NO3–N concentrations. High concentrations of heavy metals were recorded in PB and Q, probably due to the piping of road runoff directly into the stream. Generally, higher pollutant concentrations were recorded in the first storm events after the summer drought due to the flushing of accumulated solutes and a lower dilution effect, with Nk-N and NH4–N exceeding water quality standards. Over the wet season, increasing soil moisture favoured greater flow connectivity between runoff processes from pollutant sources and the stream network, leading to a higher proportion of samples exceeding pollution thresholds. No direct relationship was identified between urban extent and water quality, possibly due to the overriding impact of different storm drainage systems and flow connectivities of different urban patterns. Hydrological regime, linked to seasonal changes, also exerted a major influence on the water quality dynamics. Information on the spatiotemporal dynamics of pollutants, linked to different urban patterns and storm drainage systems, should help enable urban planners to minimize the adverse impacts of urbanization on aquatic ecosystems.