Long-term estimates of natural source zone depletion (NSZD) rates for petroleum LNAPL (light non-aqueous phase liquid) sites are not available. One-off measurements are often thought valid over the lifetime of LNAPL sites. In the context of site-wide LNAPL mass estimates, we report site-specific gasoline and diesel NSZD rates spanning 21-26 years. Using depth profiles of soil gases (oxygen, carbon dioxide, methane, volatiles) above LNAPL, NSZD rates were estimated in 1994, 2006 and 2020 for diesel and 1999, 2009 and 2020 for gasoline. Each date also had soil-core mass estimates, which together with NSZD rates allow estimation of the longevity for LNAPL presence. Site-wide coring (in 1992, 2002, 2007) estimated LNAPL mass reductions of 12,000 t. For diesel NSZD, the ratio of NSZD rates for 2006 (16,000-49,000 L/ha/y) to those in 2020 (2600-14,000 L/ha/y) was ~3-6. By 2020, the 1994 diesel NSZD rates would have predicted the entire removal of measured mass (16-42 kg/m2). For gasoline, NSZD rates in 1999 were extremely high (50,000-270,000 L/ha/y) but 9-27 times lower (5800-10,000 L/ha/y) a decade later. The gasoline NSZD rates in 1999 predicted near complete mass removal in 2-12 years, but 10-11 kg/m2 was measured 10 and 21 years later which is 26% of the initial mass in 1999. The outcomes substantiate the need to understand NSZD rate changes over the lifetime of LNAPL-impacted sites.
The chemical properties of deep profile samples (up to 12 m) of Ferrosols from northern Queensland were investigated to provide an understanding of the accumulation of nitrate (NO3) within these soil profiles. The influence of other cations and anions present in the soil solution or on the exchange and the charge chemistry of the profiles were examined with respect to the NO3 accumulations. The major ions in the soil solution were Na, NO3, and chloride (Cl). Distinct regions of anion accumulation were observed; SO4 accumulated in the upper profile of all cores, whereas NO3 and Cl accumulations were restricted to the lower profile of cores with appreciable AEC (>1�cmolc/kg). Gaines-Thomas selectivity coefficients were used to indicate exchange preference for cations and anions, and are as follows: Al > Ca ~ Mg > K > Na and sulfate (SO4) > Cl ~ NO3. The selectivity of SO4 increased and the extractable SO4 decreased in the lower profile of all cores. This has important implications for the adsorption of NO3 and Cl. The NO3 and Cl accumulations were shown to correspond to a region of low SO4 occupancy of the exchange sites in the lower profile. Along with the high SO4 selectivity, this suggests that SO4 may control the positioning of the NO3 accumulations. It was concluded that the NO3 accumulations were relatively stable under current management practices, although the reduction in NO3 inputs would likely see the gradual replacement of NO3 with Cl as a result of their comparable selectivity for exchange sites.
Petroleum biodegrades and naturally depletes. Natural Source Zone Depletion (NSZD) quantifies this at petroleum affected sites in support of management decisions for cessation of active remediation efforts. Whilst a range of NSZD estimates and methods are available, side by side comparison of NSZD rates across petroleum types in the same soil/groundwater system are lacking, especially linked to the weathering status of petroleum. At a former refinery site near Perth Western Australia, locations contaminated by crude oil, gasoline, diesel and aviation gasoline, have been intensively instrumented to enable (i) measurement of vadose zone major gas (O
[1] Streams draining urban and agriculture catchments are often a source of inorganic nutrients to downstream aquatic ecosystems, but little is known about how changes in land use influence the quality and biodegradability of dissolved organic matter (DOM). We used parallel factor analysis of excitation-emission fluorescence spectroscopy and biodegradation incubations to examine how DOM composition influences bioavailable dissolved organic carbon (DOC) in surface waters of urban and agricultural catchments during summer (low flow), winter (high flow) and spring (flow recession). Percent bioavailable DOC was variable for all catchments (2–57%) and negatively related to percent humic-like fluorescence, but positively related to percent protein-like fluorescence and simple fluorescence metrics of DOM precursor material (fluorescence index and β:α values). Conversely, highly variable DOC concentrations (2–140 mg L−1) were negatively related to protein-like fluorescence and positively related to humic-like fluorescence. Elevated concentrations of DOC (>30 mg L−1) in agro-urban streams revealed fluorescence indices (<1.3) typical of wetland and forest-dominated ecosystems, suggesting that enriched stream DOM is either derived from the destabilization of legacy soil carbon or currently produced from remnant wetlands and patches of native vegetation. Overall, we demonstrate that fluorescence characteristics can be used to predict bioavailable DOC in human-dominated catchments to better understand the flow of carbon and nutrients in aquatic food webs for improved monitoring and management of coastal ecosystems.
Abstract. The Köppen-Geiger climate classification has been used for over a century to delineate climate types across the globe. As it was developed to mimic the distribution of vegetation, it may provide a useful surrogate for making projections of the future distribution of vegetation, and hence resultant hydrological implications, under climate change scenarios. This paper developed projections of the Köppen-Geiger climate types covering the Australian continent for a 2030 and 2050 climate relative to a 1990 historical baseline climate using 17 Global Climate Models (GCMs) and five global warming scenarios. At the highest level of classification for a +2.4 °C future climate (the upper limit projected for 2050) relative to the historical baseline, it was projected that the area of the continent covered by – tropical climate types would increase from 8.8% to 9.1%; – arid climate types would increase from 76.5% to 81.7%; – temperate climate types would decrease from 14.7% to 9.2%; – cold climate types would decrease from 0.016% to 0.001%. Previous climate change impact studies on water resources in Australia have assumed a static vegetation distribution. If the change in projected climate types is used as a surrogate for a change in vegetation, then the major transition in climate from temperate to arid in parts of Australia under a drier future climate could cause indirect effects on water resources. A transition from annual cropping to perennial grassland would have a compounding effect on the projected reduction in recharge. In contrast, a transition from forest to grassland would have a mitigating effect on the projected reduction in runoff.
Nitrate (NO3) accumulations (up to 1880 kg NO3-N/ha for a 12-m profile) in the soils of the Johnstone River catchment (JRC) may pose a serious environmental threat to the Great Barrier Reef lagoon if the NO3 were released. The leaching of artificial rainwater through repacked soil columns was investigated to determine the effect of low NO3/low ionic strength inputs on the NO3 chemistry of the JRC profiles. Repacked soil columns were used to simulate the 11.5-m profiles, and the soil solution anion and cation concentrations were monitored at 10 points throughout the soil column. As the rainwater was applied, NO3 leached down the profile, with substantial quantities exiting the columns. Anion exchange was discounted as the major mechanism of NO3 release due to the substantial net loss of anions from the system (up to 2740 kg NO3-N/ha over the experimental period). As the soils were dominated by variable charge minerals, the effect of changing pH and ionic strength on the surface charge density was investigated in relation to the release of NO3 from the exchange. It was concluded that the equilibration of the soil solution with the low ionic strength rainwater solution resulted in a lessening of both the positive and negative surface charge. Nitrate was released into the soil solution and subsequently leached due to the lessening of the positive surface charge. Loss of NO3 from the soil profile was slow, with equivalent field release times estimated to be tens of years. Although annual release rates were high in absolute terms (up to 175 kg NO3-N/ha.year), they are only slightly greater than the current loss rates from fertilised sugarcane production (up to 50 kg NO3-N/ha.year). In addition to this, the large-scale release of NO3 from the accumulations will only occur until a new equilibrium is established with the input rainwater solution.
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