Christoph Hinz

Moving beyond heterogeneity and process complexity: A new vision for watershed hydrology

Field studies in watershed hydrology continue to characterize and catalogue the enormous heterogeneity and complexity of rainfall runoff processes in more and more watersheds, in different hydroclimatic regimes, and at different scales. Nevertheless, the ability to generalize these findings to ungauged regions remains out of reach. In spite of their apparent physical basis and complexity, the current generation of detailed models is process weak. Their representations of the internal states and process dynamics are still at odds with many experimental findings. In order to make continued progress in watershed hydrology and to bring greater coherence to the science, we need to move beyond the status quo of having to explicitly characterize or prescribe landscape heterogeneity in our (highly calibrated) models and in this way reproduce process complexity and instead explore the set of organizing principles that might underlie the heterogeneity and complexity. This commentary addresses a number of related new avenues for research in watershed science, including the use of comparative analysis, classification, optimality principles, and network theory, all with the intent of defining, understanding, and predicting watershed function and enunciating important watershed functional traits.

Low Pore Connectivity Increases Bacterial Diversity in Soil

ABSTRACT One of soil microbiologys most intriguing puzzles is how so many different bacterial species can coexist in small volumes of soil when competition theory predicts that less competitive species should decline and eventually disappear. We provide evidence supporting the theory that low pore connectivity caused by low water potential (and therefore low water content) increases the diversity of a complex bacterial community in soil. We altered the pore connectivity of a soil by decreasing water potential and increasing the content of silt- and clay-sized particles. Two textures were created, without altering the chemical properties or mineral composition of the soil, by adding silt- and clay-sized particles of quartz to a quartz-based sandy soil at rates of 0% (sand) or 10% (silt+clay). Both textures were incubated at several water potentials, and the effect on the active bacterial communities was measured using terminal restriction fragment length polymorphism (TRFLP) of bacterial 16S rRNA. Bacterial richness and diversity increased as water potential decreased and soil became drier (P < 0.012), but they were not affected by texture (P > 0.553). Bacterial diversity increased at water potentials of ≤2.5 kPa in sand and ≤4.0 kPa in silt+clay, equivalent to ≤56% water-filled pore space (WFPS) in both textures. The bacterial community structure in soil was affected by both water potential and texture (P < 0.001) and was correlated with WFPS (sum of squared correlations [δ2] = 0.88, P < 0.001). These findings suggest that low pore connectivity is commonly experienced by soil bacteria under field conditions and that the theory of pore connectivity may provide a fundamental principle to explain the high diversity of bacteria in soil.

Assessing the impact of regional rainfall variability on rapid pesticide leaching potential

The timing and magnitude of rainfall events are known to be dominant controls on pesticide migration into streams and groundwater, by triggering rapid flow processes, such as preferential flow and surface runoff. A better understanding of how regional differences in rainfall impact rapid leaching risk is required in order to match the scale at which water regulation occurs. We estimated the potential amount of rapid leaching, and the frequencies of these events in a case study of the southwest of Western Australia, for one soil type and a range of linearly sorbing, first order degrading chemicals. At the regional scale, those chemicals with moderate sorption and long half lives were the most susceptible to rapid transport within a year of application. Within the region, this susceptibility varied depending upon application time and seasonality in storm patterns. Those chemicals and areas with a high potential for rapid transport on average, also experience the greatest inter-annual variability in rapid leaching, as measured by the coefficient of variation. The timing and frequencies of rapid leaching events appeared to strongly relate to an areas relative susceptibility to rapid leaching. In the study region the results also suggested that frontal rainfall dominates rapid leaching along the western and southern coasts while convective thunderstorms play a greater role in the arid east.

A preferential flow leaching index

The experimental evidence suggests that for many chemicals surface runoff and rapid preferential flow through the shallow unsaturated zone are significant pathways for transport to streams and groundwater. The signature of this is the episodic and pulsed leaching of these chemicals. The driver for this transport is the timing and magnitude of rainfall events which trigger rapid flow and the release of solute from a source zone, located near the soil surface. Based on these considerations we develop a conceptual model capable of reproducing many of the signatures of this rapid transport. This driver-source-trigger-signature framework forms the basis of the development of a new leaching index which describes the potential for rapid solute transport by preferential flow or surface runoff. This preferential flow (PF) index is based upon soil and chemical parameters as well as the timing and magnitude of rainfall and preferential flow events. The PF index suggests that a chemicals potential to experience rapid transport increases as sorption strength increases, however, when an approximation to account for sorption kinetics is considered the PF index peaks at moderate sorption values. The model is sensitive to the timing and magnitude of rapid flow events, which may require existing data or infiltration models for their estimation.

Dispersion and re-deposition of fine particles and their effects on saturated hydraulic conductivity

Measurements of saturated hydraulic conductivity (K) have been used to study the rates of structural breakdown and pore clogging which occurred during leaching of 2 contrasting soils, Balkuling agricultural soil and a mining residue. Soil columns were leached with solutions of different electrolyte concentrations under saturated conditions using a Marriotte constant-head device. Measurements were made with either abrupt change of concentration from 500 to 1 mmol/L or gradual decreasing concentration from 500 through 100, 50, 10, to 1 mmol/L of NaCl followed by deionised water. The relative hydraulic conductivity (K/Ko) decreased substantially with time and with decrease in electrolyte concentrations for both soils. The decreases were attributed to various extents of internal swelling, dispersion, and decreases of pore radii as a result of detachment followed by re-deposition of the clay fraction during leaching. There was little difference in ultimate reductions in permeability between the abrupt and gradual changes of concentration from 500 to 1 mmol/L for both soils. The mining residue was clearly more prone to structural deterioration than the agricultural soil. Whereas the effluent particles from the Balkuling soil included domains or aggregates of clay crystals, those from the mining residue columns appeared to consist essentially of primary clay crystals indicating the less cohesive nature of this disturbed material. We also estimated the extent of permeability reductions using theoretical investigations based on particle size distribution. Predicted simulations agreed reasonably well with experimental data for both soils following column leaching with 1 mmol/L, with the best agreement observed for the mining residue.

In situ neutralisation of uncarbonated bauxite residue mud by cross layer leaching with carbonated bauxite residue mud

Unameliorated residue mud from the Bayer process generates highly alkaline leachates (pH ca. 13) after deposition in storage areas. Pre-deposition treatment of bauxite residue mud (BRM) with CO(2) gas (carbonation) lowers leachate pH to ca. 10.5. Laboratory scale leaching columns were used to investigate the potential for in situ pH reduction in existing uncarbonated BRM deposits through exposure to carbonated mud leachate. Leachates from uncarbonated and carbonated residues in single and dual-layer column configurations were analysed for pH, electrical conductivity, carbonate and bicarbonate content, and element concentrations. Air-dried solids were analysed by X-ray diffraction before and after leaching. Cross layer leaching lowers leachate pH from uncarbonated BRM. Leachate pH was significantly lower in dual layer and carbonated residue than in uncarbonated residue between one and 400 pore volumes leached. Carbonated residue porewater as well as dawsonite and calcite dissolution were identified as sources of (bi-)carbonate. Leachate concentrations of As, Cr, Cu, Ga and La were immediately reduced in dual layer treatments compared with uncarbonated residue. No element analysed exhibited a significantly higher leachate concentration in dual layer treatments than the highest observed concentration in single layer treatments. The implementation of dual layer leaching in the field therefore presents an opportunity to improve leachate quality from existing uncarbonated residue deposits and justifies further testing at field scale.

Using radius frequency distribution functions as a metric for quantifying root systems: Root radius frequency distributions

Root radius frequency distributions have been measured to quantify the effect of plant type, environment and methodology on root systems, however, to date the results of such studies have not been synthesised. We propose that cumulative frequency distribution functions can be used as a metric to describe root systems because (1) statistical properties of the frequency distribution can be determined, (2) the parameters for these can be used as a means of comparison, and (3) the analytical expressions can be easily incorporated into models that are dependent upon root geometry. We collated a database of 96 root radii frequency distributions and botanical and methodology traits for each distribution. To determine if there was a frequency distribution function that was best suited to root radii measurements we fitted the exponential, Rayleigh, normal, log-normal, logistic and Weibull cumulative distribution functions to each distribution in our database. We found that the log-normal function provided the best fit to these distributions and that none of the distribution functions was better or worse suited to particular shapes. We derived analytical expressions for root surface and volume and found that they are a valid, and simpler method for incorporating root architectural traits into analytical models. We also found that growth habit and growth media had a significant effect on mean root radius.

Influence of sodium adsorption ratio on sodium and calcium breakthrough curves and hydraulic conductivity in soil columns

The paper examines the effects of electrolyte concentration and sodium adsorption ratio (SAR) on the relative saturated hydraulic conductivity (RHC) and the ionic behaviour of calcium (Ca) and sodium (Na) ions in the Na-Ca exchange complex. Batch binary exchange and saturated column transport experiments were carried out to quantify these effects using an agricultural Balkuling soil and a mining residue. Generally, RHC has been found to decrease with time, with increasing SAR, and with decreasing electrolyte concentration. The more rapid decrease in RHC in the mining residue, particularly at the lowest concentration (1 mmol/L), was consistent at all SAR values. The decreases in RHC were likely to be caused by partial blocking of pores by dispersed clay particles, as evidenced by the appearance of suspended clay particles in the effluent during leaching. Significant differences in RHC were observed in the passage of fronts of decreasing electrolyte concentrations for CaCl2 and SAR 15 solutions through the soil columns. These differences were attributable to structural alterations (slaking) of the media and the nature of the particles released and mobilised within the porous structure at any given point in the column. Measurements at the critical threshold concentration and turbidity concentration at SAR 15 revealed structural breakdown of the pore matrix system as evidenced by decreased RHC. The increase in SAR to 15 is initially accompanied by erratic RHC, presumably due to the break up of soil aggregates under the increased swelling forces. The less coherent mining residue soil was substantially more vulnerable to blockage of pores than the Balkuling soil in which clay particles are likely to be more readily mobilised, and hence available to re-deposit and occlude the matrix pores.

Reactive transport controls on sandy acid sulfate soils and impacts on shallow groundwater quality

Disturbance or drainage of potential acid sulfate soils (PASS) can result in the release of acidity and degradation of infrastructure, water resources, and the environment. Soil processes affecting shallow groundwater quality have been investigated using a numerical code that integrates (bio)geochemical processes with water, solute, and gas transport. The patterns of severe and persistent acidification (pH < 4) in the sandy, carbonate-depleted podzols of a coastal plain could be reproduced without calibration, based on oxidation of microcrystalline pyrite after groundwater level decrease and/or residual groundwater acidity, due to slow vertical solute transport rates. The rate of acidification was limited by gas phase diffusion of oxygen and hence was sensitive to soil water retention properties and in some cases also to oxygen consumption by organic matter mineralization. Despite diffusion limitation, the rate of oxidation in sandy soils was rapid once pyrite-bearing horizons were exposed, even to a depth of 7.5 m. Groundwater level movement was thus identified as an important control on acidification, as well as the initial pyrite content. Increase in the rate of Fe(II) oxidation lead to slightly lower pH and greater accumulation of Fe(III) phases, but had little effect on the overall amount of pyrite oxidized. Aluminosilicate (kaolinite) dissolution had a small pH-buffering effect but lead to the release of Al and associated acidity. Simulated dewatering scenarios highlighted the potential of the model for risk assessment of (bio)geochemical impacts on soil and groundwater over a range of temporal and spatial scales.

Terrestrial exposure of oilfield flowline additives diminish soil structural stability and remediative microbial function

Onshore oil production pipelines are major installations in the petroleum industry, stretching many thousands of kilometres worldwide which also contain flowline additives. The current study focuses on the effect of the flowline additives on soil physico-chemical and biological properties and quantified the impact using resilience and resistance indices. Our findings are the first to highlight deleterious effect of flowline additives by altering some fundamental soil properties, including a complete loss of structural integrity of the impacted soil and a reduced capacity to degrade hydrocarbons mainly due to: (i) phosphonate salts (in scale inhibitor) prevented accumulation of scale in pipelines but also disrupted soil physical structure; (ii) glutaraldehyde (in biocides) which repressed microbial activity in the pipeline and reduced hydrocarbon degradation in soil upon environmental exposure; (iii) the combinatory effects of these two chemicals synergistically caused severe soil structural collapse and disruption of microbial degradation of petroleum hydrocarbons.