Assessment of suburban stormwater retention ponds in Southern Finland

Abstract

This study examined how changes in land use and stormwater network affected pollutant loads and capacities of stormwater retention ponds in Lahti, Finland during the urban development of the area. The focus was on the Hennala district, where three retention ponds had been constructed to manage stormwater runoff and improve water quality. The study aimed to analyze land use and runoff volume changes over time, estimate pollutant loads, and propose strategies to enhance the stormwater management practices at the ponds and the catchment.

By employing the volume-concentration methodology, combining land use and stormwater network data, aerial imagery and digital elevation models (DEMs), the study quantified the increase in pollutant loads due to urbanization, primarily driven by the expansion of impervious surfaces over an extended period of time. In the catchment, non-built-up areas decreased by over 50% in the past 60 years. From 1979 to 2022, the load of total suspended solids (TSS) increased by 51%, while the loads of chemical oxygen demand (COD), total nitrogen (TN), and total phosphorus (TP) increased by 33%, 21%, and 44%, respectively. Additionally, heavy metal levels, including lead (Pb), zinc (Zn), copper (Cu), chromium (Cr), and nickel (Ni), experienced rises of 37%, 60%, 60%, 54% and 57%, respectively. Subcatchments with extensive industrial and commercial development exhibited the most pronounced pollutant load increases. The retention ponds, although initially designed to manage the polluted stormwater, became overloaded by the increased runoff and pollution loads. The 2012 stream diversion distributed the loads more evenly to the ponds, but the overall system remains vulnerable to future challenges.

The results highlighted the need for proactive measures to enhance the capacity and efficiency of stormwater retention ponds in Lahti. Future research should focus on refining pollutant load estimation, incorporating local hydrological and meteorological variability, and exploring alternative stormwater management solutions. The current results also provided insights for urban planners and decision-makers in areas similar to the study.