River Water-quality Trends Research Articles

Trend in river water quality: tracking the overall impacts of climate change and human activities on water quality in the Dez River Basin

Abstract A perceptible degradation in water quality complicates safe water supply for drinking and irrigation purposes. Therefore, this study aims at monitoring water quality changes and effective factors in the Dez River Basin, which are required to manage water resources effectively. To this end, the common influence of flow rate changes on water quality was separated by implementing seasonal Mann–Kendall test on residuals resulting from the LOWESS test. The results show that after adjusting the effects of seasonality and streamflow fluctuations, significant positive trends in most water quality parameters are still observed. It emphasizes the role of other factors controlling river water quality in the basin. Comparison of the trends of modified quality parameter time series (residuals) in different subbasins having natural or mad-made conditions, with or without significant groundwater resources, shows almost the same presented trends in water quality. This supports that, overall, minor changes occurred in land use, groundwater table, and environmental and human factors with no important influences on presented trends in water quality. Our analyses show that overall reduction in precipitation as well as positive trends in temperature and evaporation led to intensified streamflow variations, explaining the main changes in the river water quality of the basin.

An approach for decomposing river water-quality trends into different flow classes

A number of statistical approaches have been developed to quantify the overall trend in river water quality, but most approaches are not intended for reporting separate trends for different flow conditions. We propose an approach called FN2Q, which is an extension of the flow-normalization (FN) procedure of the well-established WRTDS (“Weighted Regressions on Time, Discharge, and Season”) method. The FN2Q approach provides a daily time series of low-flow and high-flow FN flux estimates that represent the lower and upper half of daily riverflow observations that occurred on each calendar day across the period of record. These daily estimates can be summarized into any time period of interest (e.g., monthly, seasonal, or annual) for quantifying trends. The proposed approach is illustrated with an application to a record of total nitrogen concentration (632 samples) collected between 1985 and 2018 from the South Fork Shenandoah River at Front Royal, Virginia (USA). Results show that the overall FN flux of total nitrogen has declined in the period of 1985–2018, which is mainly attributable to FN flux decline in the low-flow class. Furthermore, the decline in the low-flow class was highly correlated with wastewater effluent loads, indicating that the upgrades of treatment technology at wastewater treatment facilities have likely led to water-quality improvement under low-flow conditions. The high-flow FN flux showed a spike around 2007, which was likely caused by increased delivery of particulate nitrogen associated with sediment transport. The case study demonstrates the utility of the FN2Q approach toward not only characterizing the changes in river water quality but also guiding the direction of additional analysis for capturing the underlying drivers. The FN2Q approach (and the published code) can easily be applied to widely available river monitoring records to quantify water-quality trends under different flow conditions to enhance understanding of river water-quality dynamics.

Water quality trends in the last decade for ten watersheds dominated by diffuse pollution in Québec (Canada)

The aim of this work is to evaluate and discuss river water quality trends over the last decade in ten watersheds where diffuse pollution represents more than half of the annual load of phosphorus (P) and nitrogen (N). Trend analyses taking into account flow data indicate a significant reduction of total P in eight rivers, of ammonia N in five rivers, of nitrate + nitrite in four rivers, of total filtered N in three rivers and of suspended solids in two rivers. An increase of turbidity was observed in four rivers and, for fecal coliforms, no trends. P decrease can be explained by reduced mineral P inputs on cropped lands related to means such as agro-environmental fertilization plans and addition of phytase in pig and poultry feed. However, for seven of them, median P concentrations remain at least two times greater than the Québec water quality guideline for protection of rivers against eutrophication. Concentrations of other parameters remain problematic in some rivers too. These results indicate the need to continue the efforts for further diffuse pollution reduction. Future work should better quantify actions taken at the watershed scale to reduce diffuse pollution.