Loess: The Yellow Earth

A wind-deposited silt forming large deposits in China and middle America, loess is the basis of much grade-one agricultural land.     

Palaeohydraulics revisited: palaeoslope estimation in coarse-grained braided rivers

Methods for estimating palaeoslope from fluvial deposits have been available for some time, but new data and improved understanding of the relevant physical processes afford the possibility of improving existing methods, and the emerging field of quantitative stratigraphy provides a new context for the results. Here we focus on deriving palaeoslope estimates for coarse-grained fluvial deposits. These estimates can be used in basin analyses to constrain the magnitude of the slope change necessary for a given deflection of palaeocurrents, to constrain temporal and spatial variation in basin subsidence rate, and to provide a surface datum for use in sediment-backstripping calculations. The algorithm we derive to estimate palaeoslope applies to rivers that self-adjust through variations in channel width to maintain a temporally and spatially averaged bed shear stress equal to some constant multiple of the critical shear stress for initial motion of bed sediment. Data from modern coarse-grained rivers with minimal bank cohesion and form resistance suggest that this boundary shear stress is equal to about 1.4 times the critical shear stress for movement of the median-sized clast of the surface layer. The key sedimentological criteria for recognition of systems appropriate for this type of analysis are: (1) field relations suggesting that channel banks formed in effectively noncohesive gravel (i.e. free of clay-size sediment and plant roots); (2) absence of significant volumes of dune-derived cross-stratification; and (3) absence of indicators of extremely rapid, flash-flood-type deposition. The basic input data for a palaeoslope calculation are spatially averaged estimates of palaeodepth and median grain size. The most important aspect of data collection is that the depth and grain-size estimates should be determined independently by random sampling over the whole outcrop. Joint analysis of data from appropriate modern rivers and of errors associated with palaeodepth and grain-size estimates indicates that in coarse-grained braided-river deposits, palaeoslope can be estimated to within a factor of 2.     

Crustal accretion in the Pan African: Nd and Sr isotope evidence from the Arabian Shield

Nd and Sr isotope determinations on late Precambrian to early Palaeozoic igneous and sedimentary rocks from the Arabian Shield are used to investigate the proportion of reworked “older” crust, and the rate at which new crust was generated during the Pan African event. Eight Rb/Sr whole rock isochrons on igneous suites yield ages in the range 770?590 Ma and initial ⁸⁷Sr/⁸⁶Sr ratios of 0.7038?0.7023. These data confirm that magmatism in this area was largely restricted to the period 850-550 Ma, and the initial ratios are sufficiently low to preclude significant contributions from a long-lived upper crustal source. The initial ¹⁴³Nd/¹⁴⁴Nd ratios of a variety of lithologies, including several samples of possible “basement”, are all higher than the contemporaneous values for CHUR (ε_Nd = +1.6 to +6.9), suggesting that many were derived directly from the upper mantle, and that any inferred crustal source regions for the remainder could not have separated from likely LREE-depleted mantle reservoirs before 1200 Ma. The Arabian Shield therefore provides an example of rapid crustal growth during the Late Proterozoic, and contrasts with the Damara intracratonic belt of Namibia where Nd and Sr isotopes provide strong evidence for extensive reworking of older continental crust during the same period.     

Comparison of Two Integrated Methods for the Collection and Analysis of Volatile Organic Compounds in Ground Water

The principal difficulties with determinations of volatile organic compounds (VOCs) in ground water are the reliability of sampling procedures and analytical methods. Two integrated methods have been developed for routine sampling, processing, and analysis of VOCs in ground water. These methods involve in situ collection of ground water using a modified syringe sampler from PVC piezometers or using dedicated glass syringes from stainless steel multilevel bores. The samples are processed in the syringe using purge and trap or microsolvent extraction and analyzed by GC/MSD.
The modified purge-and-trap method is time-consuming and limited to volatile organic compounds. However, it is extremely sensitive and flexible: the volume of sample used can be varied by the use of different-size glass syringes (sample volumes from 1 to 100 mL).
In cases where extremely low sensitivity (<10 mg 1⁻¹) is not critical, the microextraction technique is a more cost-effective method, allowing twice as many samples to be analyzed in the same time as the purge-and-trap method. It enables less volatile compounds such as polynuclear aromatic hydrocarbons, phenol, and cresols to be analyzed in the same GC run. Also, the microextraction method can be used in the field to avoid delays associated with transportation of ground water samples to the laboratory.     

Observation of Conditions Preceding Peak Indoor Air Volatile Organic Compound Concentrations in Vapor Intrusion Studies

Temporal and spatial variability of indoor air volatile organic compound (VOC) concentrations can complicate vapor intrusion (VI) assessment and decision-making. Indicators and tracers (I&T) of VI, such as differential temperature, differential pressure, and indoor radon concentration, are low-cost lines of evidence to support sampling scheduling and interpretation of indoor air VOC sampling results. This study compares peak indoor air chlorinated VOC concentrations and I&T conditions before and during those peak events at five VI sites. The sites differ geographically and in their VI conceptual site models (CSM). Relative to site-specific baseline values, the results show that cold or falling outdoor temperatures, rising cross slab differential pressures, and increasing indoor radon concentrations can predict peak VOC concentrations. However, cold outdoor air temperature was not useful at one site where elevated shallow soil temperature was a better predictor. Correlations of peak VOC concentrations to elevated or rising barometric pressure and low wind speed were also observed with some exceptions. This study shows how the independent variables that control or predict peak indoor air VOC concentrations are specific to building types, climates, and VI CSMs. More I&T measurements at VI sites are needed to identify scenario-specific baseline and peak related I&T conditions to improve decision-making.     

Headwaters drive streamflow and lowland tracer export in a large-scale humid tropical catchment

Headwaters are generally assumed to contribute the majority of water to downstream users, but how much water, of what quality and where it is generated are rarely known in the humid tropics. Here, using monthly monitoring in the data scarce (2,370 km²) San Carlos catchment in northeastern Costa Rica, we determined runoff-area relationships linked to geochemical and isotope tracers. We established 46 monitoring sites covering the full range of climatic, land use and geological gradients in the catchment. Regression and cluster analysis revealed unique spatial patterns and hydrologically functional landscape units. These units were used for seasonal and annual Bayesian tracer mixing models to assess spatial water source contributions to the outlet. Generally, the Bayesian mixing analysis showed that the chemical and isotopic imprint at the outlet is throughout the year dominated by the adjacent lowland catchments (68%) with much less tracer influence from the headwaters. However, the headwater catchments contributed the bulk of water and tracers to the outlet during the dry season (>50%) despite covering less than half of the total catchment area. Additionally, flow volumes seemed to be linearly scaled by area maintaining a link between the headwaters and the outlet particularly during high flows of the rainy season. Stable isotopes indicated mean recharge elevations above the mean catchment altitude, which further supports that headwaters were the primary source of downstream water. Our spatially detailed “snap-shot” sampling enabled a viable alternative source of large-scale hydrological process knowledge in the humid tropics with limited data availability.     

Lessons from the 2018 drought for management of local water supplies in upland areas: A tracer-based assessment

Climate change, combined with industrial growth and increasing demand, could result in serious future water shortages and related water quality and temperature issues, especially for upland and humid areas. The extreme 2018 drought that prevailed throughout Europe provided an opportunity to investigate conditions likely to become more frequent in the future. For an upland rural catchment utilised by the distilling industry in North-East Scotland, a tracer-based survey combined discharge, electrical conductivity, stable water isotopes and temperature measurements to understand the impacts of drought on dominant stream water and industry water sources, both in terms of water quantity and quality (temperature). Results showed that water types (groundwater, ephemeral stream water, perennial stream water and water from small dams) were spatially distinct and varied more in space than time. With regards to the drought conditions we found that streams were largely maintained by groundwater during low flows. This also buffered stream water temperatures. Water types with high young water fractions were less resilient, resulting in streams with an ephemeral nature. Although our results demonstrated the importance of groundwater for drought resilience, water balance data revealed these storage reserves were being depleted and only recovered towards the end of the following year because of above average rainfall in 2019. Increased storage depletion under continued trends of extreme drought and water abstraction could be addressed via informed (nature based) management strategies which focus on increasing recharge. This may improve resilience to droughts as well as floods, but site specific testing and modelling are required to understand their potential. Results could have implications for management of water volumes and temperature, particularly for the sustainability of an historic industry, balancing requirements of rural communities and the environment.