Mapping and analysing virtual outcrops

Laser scanning is a very efficient way to generate realistic, high-resolution digital models of 3-D geological outcrops. This paper discusses the methodologies involved in the creation and analysis of virtual outcrops, based on laser scanner data. The visualisation of the laser scanner data as a photorealistic 3-D object is described. Geological features picked out on the virtual outcrop (e.g. fractures, faults or bedding planes) can be extrapolated outward, into space, and inward, into the subsurface, using tension surfaces.Electronic Supplementary Material Supplementary material is available in the online version of this article at Reviewed by: J.D. Clemens, D. Yuen     

Internal erosion of soil pipes: Sediment rating curves for soil pipes

The collapse of soil pipes due to internal erosion can result in fully mature gullies. Few studies have measured the rates of sediment detachment and transport through soil pipes in situ. The objectives of this work were to determine suspended sediment concentration (SSC) in soil pipes as a function of pipeflow rate to develop sediment rating curves (SRC) and measure the bedload transport as a function of cumulative flow per storm event. H-flumes were installed in seven discontinuous gullies formed by pipe collapse and instrumented for pipe discharge measurements and suspended sediment sampling. The typical response to pipeflow was an initial flush of high concentration of suspended sediment followed by a decrease as pipeflow increased (rising limb of hydrograph). Pipeflows were often so dynamic that it was difficult to consistently capture the initial flush of sediment, resulting in weak to non-existent SRCs. The falling limb of the hydrograph tended to have a relatively low SSC. Thus, soil pipe SRCs tended to be better represented by hysteretic SRCs, although relationships between SSC and flow rate were poorly represented by SRCs. A power law equation given by SSC = aQ^b was adopted to represent the SRC relationships. Fitting this equation to data showed a correlation between the offset, a, and the slope, b, with the slope decreasing as the offset increases. Both SRC parameters (a and b) were correlated to the contributing area of the individual pipe. Bedload appeared to be an important contributor to sediment transport, with bedload – expressed as an average event sediment concentration (mg l⁻¹) – decreasing as the volume of the event discharge (m³) increased. A significant portion (11–31%) of the bedload material was gravel and aggregates (>2 mm diameter material). While this work was the first to determine SRCs for soil pipes, refined sampling and measurement techniques are needed. © 2020 John Wiley & Sons, Ltd.     

Patterns,puzzles and people: implementing hydrologic synthesis

There have been several calls made for hydrologic synthesis research: namely activities which unify diverse data sources across sites, scales and disciplines to uncover new connections and to promote a holistic understanding of water science. This paper draws on the NSF‐funded Hydrological Synthesis Project (HSP) run by the University of Illinois at Urbana‐Champaign to elucidate mechanisms, benefits and challenges of implementing hydrologic synthesis research from the perspectives of participants in a pilot research study. Two broadly different mechanisms of implementing synthesis were adopted in the HSP: 6‐week Summer Institutes in which Ph.D. students conducted team‐based research under the guidance of faculty mentors, and focused workshops which disseminated knowledge and shared experiences between scientists at many different career levels. The Summer Institutes were a test bed in which new ideas could be explored, assisted students in developing a wide range of skills, and were highly productive, but posed challenges for mentors and students because the ‘new’ research topics initiated during the Institutes' programmes needed to be completed in competition with students' ongoing Ph.D. research or mentor's existing research programs. The workshop‐based model circumvented this conflict and was also highly productive, but did not offer the same opportunity to experiment with new ideas as part of the synthesis research. Leadership, trust, flexibility and long gestation times were all important to bringing synthesis research to a positive resolution. Funding models that embrace the exploratory aspects of synthesis and provide adequate support to mentors and students over these long timescales would facilitate future hydrologic synthesis research. Copyright © 2011 John Wiley & Sons, Ltd.     

Soil pipe collapses in a loess pasture of Goodwin Creek watershed,Mississippi: role of soil properties and past land use

Little is known about the association of soil pipe collapse features with soil properties or land use history. Three loess covered catchments in northern Mississippi, USA were characterized to investigate these relationships. Soil pipe collapses were characterized for their size, type feature and spatial location along with soil properties across the three catchments. Although mapped as the same soil, one of the catchments did not contain pipe collapse features while the other two had 29.4 and 15.4 pipe collapses per hectare. These loess soils contained fragipan layers that are suspected of perching water, thereby initiating the piping processes. Pipe collapses associated with subsurface flow paths were not always consistent with surface topography. The surface layer tended to be non‐erodible while layers below, even the upper fragipan layers, were susceptible to erosion by pipeflow. Soil properties of the lowest fragipan layer were highly variable but tended to prevent further downward erosion of soil pipes and thus formed a lower boundary for gullies. Middle to lower landscape positions in one of the piped catchments contained anthropic soils that were highly erodible. These anthropic soils were previously gullies that were filled‐in in the 1950s when forested areas, assumed to have been established when land was previously converted from crop to forest land, were converted to pasture. Three decades after this land use change from forest to pasture, pipe collapses became evident. In contrast, the adjacent catchment that does not exhibit pipe collapse features experienced severe sheet and rill erosion prior to the 1930s while in cotton production. The surface horizons above the lower fragipan layer were completely removed during this period, thus the top‐soil layer that tends to form a bridge above soil pipes in the more erodible subsoil layers was removed. This study showed that knowledge of soil characteristics or topography alone do not explain the distribution of soil pipe collapses as past land use can play a definitive role. Copyright © 2015 John Wiley & Sons, Ltd.     

Variation of hydrogeochemical characteristics of water in surface flows,shallow wells,and boreholes in the coastal city of Douala (Cameroon)

ABSTRACT

Groundwater is used by 3?million inhabitants in the coastal urban city of Douala, Cameroon, but comprehensive data are too sparse for it to be managed in a sustainable manner. Hence this study aimed to (1) assess the potability of the groundwater; (2) evaluate the spatial variation of groundwater composition; and (3) assess the interaction and recharge mechanisms of different water bodies. Hydrogeochemical tools and methods revealed the following results in the Wouri and Nkappa formations of the Douala basin, which is beneath Douala city: 30% of water samples from hand-dug wells in the shallow Pleistocene alluvium aquifer were saline and highly mineralized. However, water from boreholes in the deeper (49–92 m depth) Palaeocene aquifer was saline-free, less mineralized and potable. Water in the shallow aquifer (0.5–22 m depth) was of Na⁺-K⁺-Cl^?-NO₃^? type and not potable due to point source pollution, whereas Ca⁺-HCO₃^? unpolluted water dominates in the deeper aquifer. Water in the deep and shallow aquifers indicates the results of preferential flow pass and evaporative recharge, respectively. Possible hydrogeochemical processes include point source pollution, reverse ion exchange, remote recharge areas and mixing of waters with different chemical signatures.

EDITOR D. Koutsoyiannis ASSOCIATE EDITOR M.D. Fidelibus     

Efficacy of an In‐Well Sonde to Determine Magnetic Susceptibility of Aquifer Sediment

Magnetite is a natural component of many aquifers. Abiotic degradation of chlorinated solvents by magnetite can be an important mechanism for natural attenuation of these contaminants. The quantity of magnetite in aquifer materials can be estimated by measuring the magnetic susceptibility of the materials. This is most commonly done by determining the magnetic susceptibility of core samples in an analytical laboratory using a magnetic susceptibility meter. Unfortunately, the cost of acquiring core samples often makes an evaluation of abiotic degradation by magnetite economically unrealistic. Downhole sondes (probes) are available for the determination of magnetic susceptibility. In this study, a downhole sonde was evaluated as an affordable alternative to acquiring and analyzing core samples. The sonde was introduced into 10 monitoring wells. The data from the sonde were then compared to data from core samples that were collected from the same elevation as the sonde data. The core samples analyzed in the laboratory were used as the standard against which the sonde data were compared. The downhole sonde reported values that were similar to values reported on core samples. At most wells, the means of the two measurements could not be distinguished at the 95% confidence interval. When the means could be distinguished, they still agreed within a factor of two.     

Interface Treatment of Petroleum Hydrocarbon‐impacted Lower Permeability Layers by Activated Sodium Persulfate to Reduce Emissions to Groundwater

Nonaqueous phase liquid (NAPL)‐impacted lower permeability layers in heterogeneous media are difficult to fully remediate and can act as persistent sources of groundwater contamination through diffusive emissions to transmissive aquifer zones. This work investigated the benefits of partial remediation involving treatment focused near the high‐low permeability interface, with the performance metric being emissions reduction. A sequential base‐activated persulfate (S₂O₈ ^2?) delivery treatment strategy was studied in this work, involving 13–14 d deliveries of 10% w/w sodium persulfate (Na₂S₂O₈) and 14–28 d deliveries of 19 g/L sodium hydroxide (NaOH). Treatment and control experiments were conducted in 1.2‐m wide × 1.2‐m tall × 5‐cm thick physical model tanks containing two soil layers differing in hydraulic conductivity by three orders of magnitude. The top 10 cm of the lower permeability layers contained 7400–7800 mg‐NAPL/kg‐soil; the NAPL was comprised of benzene, toluene, ethylbenzene, p‐xylene, o‐xylene, n‐propylbenzene, and 1,3,5‐trimethylbenzene (TMB) mixed in octane. Approximately 0.1 g‐Na₂S₂O₈ was delivered per cm²‐interface area over 13–14 d. The S₂O₈ ^2? and SO₄ ^2? concentration profiles suggest higher oxidant reaction rates when NaOH is delivered prior to, rather than after Na₂S₂O₈. After 264 d and two treatments, hydrocarbon emissions from the NAPL source were reduced by 60% to 73% compared to a no‐treatment control tank. The incremental benefit of the second treatment was only about 10% of the effect of the first treatment.     

Remediation Decision‐Making and Behavioral Economics: Results of an Industry Survey

Decision methods applied in the remediation industry were evaluated using a survey of industry practitioners, in order to assess the relative roles of quantitative decision analysis and gut intuition. Principals from the disciplines of behavioral economics and decision theory were used as a framework to evaluate remediation decision behaviors revealed by the survey. The survey was completed by 118 respondents representing academia, consultants, clients, and others. Survey questions focused on perceptions and experiences related to inputs to decisions and decision processes, as well as remediation goal setting and outcomes. The most common remediation objective cited was short‐term interim measures and the least common was no further action (NFA) with clean closure. NFA was also sparingly achieved: 33% of respondents reported zero NFA closures in their career, and an overall 15% to 20% NFA closure rate was reported among more experienced respondents. Data inputs were ranked most important to decisions, while the decision process itself was ranked lowest. Intuition‐based decision methods such as asking for a trusted opinion, rules of thumb, and meetings were all used at last twice as often as decision analysis such as discounted cash flow or probabilistic analysis. Analysis of survey responses showed that cognitive biases, including overconfidence effect and intuition bias, are present to some extent in remediation decision‐making. Practitioners are advised to be mindful of the decision‐making processes they apply, and to incorporate elements of both intuition and decision analysis, as appropriate to the decisions being made.     

Stability of a circular tunnel in cohesive-frictional soil subjected to surcharge loading

The stability of circular tunnels in cohesive-frictional soils subjected to surcharge loading has been investigated theoretically and numerically assuming plane strain conditions. Despite the importance of this problem, previous research on the subject is very limited. At present, no generally accepted design or analysis method is available to evaluate the stability of tunnels/openings in cohesive-frictional soils. In this study, continuous loading is applied to the ground surface, and both smooth and rough interface conditions are modelled. For a series of tunnel diameter-to-depth ratios and material properties, rigorous lower- and upper-bound solutions for the ultimate surcharge loading are obtained by applying finite element limit analysis techniques. For practical use, the results are presented in the form of dimensionless stability charts with the actual tunnel stability numbers being closely bracketed from above and below. As an additional check on the solutions, upper-bound rigid-block mechanisms have been developed and the predicted collapse loads from these are compared with those from finite element limit analysis. Finally, an expression that approximates the ultimate surcharge load has been devised which is convenient for use by practising engineers.