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Assessment of water quality evolution in the thousands of existing and future mine pit lakes worldwide requires new numerical tools that integrate geochemical, hydrological, and biological processes. A coupled model was used to test alternative hypothesized controls on water quality in a pit lake over ∼8 years. The evolution of pH, Al, and Fe were closely linked; field observations were reproduced with generic solubility equilibrium controls on Fe(III) and Al and a commonly reported acceleration of the abiotic Fe(II) oxidation rate by 2-3 orders of magnitude. Simulations indicated an ongoing acidity loading at the site, and the depletion of Al mineral buffering capacity after ∼5 years. Simulations also supported the existence of pH limitation on nitrification, and a limitation on phytoplankton growth other than the commonly postulated P and DIC limitations. Furthermore, the model reproduced the general patterns of salinity, pH, Al, and Fe during an uncontrolled river breach in 2011, however, incorporating sediment biogeochemical feedbacks is required to reproduce the observed postbreach internal alkalinity generation in the lake. The modeling approach is applicable to the study of hydrological, geochemical, and biological interactions for a range of lake and reservoir management challenges.

Nutrient data from catchments discharging to receiving waters are monitored for catchment management. However, nutrient data are often sparse in time and space and have non-linear responses to environmental factors, making it difficult to systematically analyse long- and short-term trends and undertake nutrient budgets. To address these challenges, we developed a hybrid machine learning (ML) framework that first separated baseflow and quickflow from total flow, generated data for missing nutrient species, and then utilised the pre-generated nutrient data as additional variables in a final simulation of tributary water quality. Hybrid random forest (RF) and gradient boosting machine (GBM) models were employed and their performance compared with a linear model, a multivariate weighted regression model, and stand-alone RF and GBM models that did not pre-generate nutrient data. The six models were used to predict six different nutrients discharged from two study sites in Western Australia: Ellen Brook (small and ephemeral) and the Murray River (large and perennial). Our results showed that the hybrid RF and GBM models had significantly higher accuracy and lower prediction uncertainty for almost all nutrient species across the two sites. The pre-generated nutrient and hydrological data were highlighted as the most important components of the hybrid model. The model results also indicated different hydrological transport pathways for total nitrogen (TN) export from two tributary catchments. We demonstrated that the hybrid model provides a flexible method to combine data of varied resolution and quality and is accurate for the prediction of responses of surface water nutrient concentrations to hydrologic variability. [ABSTRACT FROM AUTHOR]

Among different Water Sensitive Urban Design options, constructed wetlands (CWs) are used to protect and restore downstream water quality by attenuating nutrients generated by stormwater runoff. This research compared the nutrient attenuation ability during a diverse population of storm events of two CWs: (a) a hybrid CW with multiple alternating surface flow (SF) and laterite-based subsurface flow (SSF) compartments, and (b) a single stage SF CW. Within-storm variability, nutrient concentrations were assessed at 2 to 3-h intervals at both the main inlet and outlet of each CW. Dissolved oxygen concentrations of the surface waters were also monitored at 10-min intervals using high frequency in situ sensors. Nutrient loads into the CWs were observed to be higher when a high rainfall event occurred, particularly after longer antecedent dry conditions. Longer hydraulic retention times promoted higher attenuation at both sites. However, the relative extent of nutrient attenuation differed between the CW types; the mean total nitrogen (TN) attenuation in the hybrid and SF CW was 45 and 48%, respectively. The hybrid CW attenuated 67% total phosphorus (TP) loads on average, while the SF CW acted as a net TP source. Periodic storm events transitioned the lentic CW into a lotic CW and caused riparian zone saturation; it was therefore hypothesized that such saturation of organic matter rich-riparian zones led to release of TP in the system. The hybrid CW attenuated the released TP in the downstream laterite-based SSF compartments. Diel oxygen metabolism calculated before and after the storm events was found to be strongly correlated with water temperature, solar exposure and antecedent dry condition during the pre-storm conditions. Furthermore, the SF CW showed a significant relationship between overall nutrient load attenuation and the change in oxygen metabolism during the storm perturbation, suggesting oxygen variation could be a useful proxy indicator of CW function. [ABSTRACT FROM AUTHOR]

[Display omitted] • A structured trend and multiple change point framework was developed, suitable for Mediterranean regions. • Monthly/annual hydrological and climatic metrics assessed concurrently. • The extent of climatic change and drying trend revealed across an extensive regional network. • Main drivers of changes and their implication to the regional management were discussed. Many regions of the world have experienced rapid hydro-climatic changes in recent decades. Mediterranean areas have especially experienced prolonged droughts, changes to runoff, and low-flow periods. Many methods and assumptions for assessing trends in rainfall and runoff are available, however, a comprehensive framework is required that can consider variations at large temporal and spatial scales, long-term persistence (LTP), seasonal patterns, and multiple change points. An analysis framework has been developed in R and applied to a network of 107 rainfall gauges (with about 100 yrs data) and 90 streamflow-water quality gauges (30 – 50 yrs data) across South-west Western Australia, a region with a typical Mediterranean climate. Trends were estimated using the Original Mann–Kendall (OMK) family of tests including using the Mann-Kendall Test under the Scaling Hypothesis (MKLTP) and Seasonal Mann–Kendall (SMK). Single and multiple change points were calculated by the non-parametric Lanzante's test (LAT), E-Agglomerative algorithm (ECP), and Energy Divisive (EDP). Trend tests show annual rainfall has decreased by 2.6 mm/yr in the Western areas and increased by 0.9 mm/yr in the Eastern areas with corresponding change points in the 1940 s-1960 s and from 1995 to 2000 although 30 % of the annual rainfall and 100 % of the standardized precipitation index (SPI) time series were found to be under LTP. A regional flow trend analysis shows catchments are yielding much less runoff in more than 90 % of gauged stations. The runoff in perennial and ephemeral streams has shown a decline with the rate of 6 (54 %) and 2 (50 %) mm/yr, respectively. Runoff and baseflow decline occurred as step changes lagged after rainfall, and the rate of decline in baseflow was more than for runoff, suggesting that groundwater connectivity with stream inverts has fallen substantially. Stream salinities increased by 64 mg/L/yr (or 15 %) overall which was highly significant. The framework provides a structured approach to analyse changes in rainfall, runoff and stream salinity for a Mediterranean climate that is projected to become even warmer and drier. [ABSTRACT FROM AUTHOR]

Wind sheltering can have a strong effect in small, sheltered water bodies; wind-induced mixing in the surface layer of a small mine lake has been investigated using field measurements. Wind speed was recorded at three locations and data suggested sheltering by topography and surface roughness changes with both fetch and the land–water transition. Wind sheltering effects in the near-surface waters were assessed using turbulent microstructure profiler measurements, providing an estimate of the sheltering distance consistent with the literature on “backward-facing” steps. A numerical simulation of the annual density stratification cycle was then performed, using the model DYRESM. Simulations indicated that inclusion of a sheltering algorithm based on the results of the field campaign significantly improved the model’s performance in capturing the surface mixed layer deepening associated with strong wind events. [ABSTRACT FROM AUTHOR]

Abstract: The injection and recovery of oxic water into deep anoxic aquifers may help to alleviate short- and long-term imbalance between freshwater supply and demand. The extent and structure of physical and geochemical heterogeneity of the aquifer will impact the water quality evolution during injection, storage and recovery. Water–sediment interactions within the most permeable parts of the aquifer, where the bulk of the injectant will penetrate, may dominate, however, water quality may also be impacted by interactions within the finer-grained, less permeable but potentially highly reactive media. In this study, the heterogeneity of the reductive capacity of an aquifer selected for water reuse projects was characterised, the amount, type and reactivity of the sedimentary reductants present determined, and the relationship between reductive capacity and sedimentary lithologies quantified. The average potential reductive capacities (PRCTOT), based on total organic C and pyrite concentrations of the sediment, were quantified for sands (382μmol O2 g−1), clays (1522μmol O2 g−1), and silts (1957μmol O2 g−1). Twenty-seven samples, spanning the three different lithologies, were then incubated for 50days and the measured reductive capacities (MRC) determined for the sands (29.2μmol O2 g−1), silts (136μmol O2 g−1), and clays (143μmol O2 g−1). On average, the MRC were 10% of the PRCTOT. The main consumers of O2 were pyrite (20–100%), sedimentary organic matter (SOM; 3–56%), siderite (3–28%) and Fe(II)-aluminosilicates (8–55%). The incubation data plus hydrogeochemical modelling, indicated that pH-buffering was controlled firstly by dissolution of trace level carbonates, followed by dissolution of feldspars. Zinc, Co, Ni, Cd and Pb were readily mobilized during incubation. [Copyright &y& Elsevier]

Effective short- and long-term estuarine water quality management decisions require a holistic view of estuarine response to multiple stressors that may be achieved through the integration of numerical modelling and observed data. Such an approach has been developed for the Swan-Canning Estuary system, a eutrophic urban estuary in Western Australia under threat from nutrient enrichment and a drying climate. Numerical modelling was integrated with long-term monitoring to develop the system Swan-Canning Estuary Virtual Observatory (SCEVO), which has been used to facilitate water quality management and streamline prediction workflows of hindcast, forecast, and environmental response functions. The system is based on a validated 3D water quality model, integrated within a data management system and related environmental models. A machine-learning method to improve the patchy and time-lagged catchment inputs is also highlighted. This work has identified that the key challenge associated with estuarine water quality prediction is the capability to (1) simulate internal physical and biogeochemical processes at suitable spatial resolution to resolve the gradients along the freshwater-ocean continuum; and (2) transition from using routine monitoring data as the basis for management decisions to using a diverse and integrated set of data streams as the basis for real-time operational decisions. Recommendations for high-frequency monitoring to support water quality modelling and dynamic integration between numerical and observed data for improved forecasting are discussed. • A real-time water quality forecasting system for an urban estuary was developed. • Machine-learning was used to fill and forecast flow and nutrient boundary conditions. • Three modelling workflows are described to meet the needs of various end-users. • Integrated data-modelling system can support short-term and long-term management. [ABSTRACT FROM AUTHOR]