Geostatistical analysis of centimeter-scale hydraulic conductivity variations at the MADE site

This is the published version. Copyright American Geophysical Union[1] Spatial variations in hydraulic conductivity (K) provide critical controls on solute transport in the subsurface. Recently, new direct-push tools were developed for high-resolution characterization of K variations in unconsolidated settings. These tools were applied to obtain 58 profiles (vertical resolution of 1.5 cm) from the heavily studied macrodispersion experiment (MADE) site. We compare the data from these 58 profiles with those from the 67 flowmeter profiles that have served as the primary basis for characterizing the heterogeneous aquifer at the site. Overall, the patterns of variation displayed by the two data sets are quite similar, in terms of both large-scale structure and autocorrelation characteristics. The direct-push K values are, on average, roughly a factor of 5 lower than the flowmeter values. This discrepancy appears to be attributable, at least in part, to opposite biases between the two methods, with the current versions of the direct-push tools underestimating K in the highly permeable upper portions of the aquifer and the flowmeter overestimating K in the less permeable lower portions. The vertically averaged K values from a series of direct-push profiles in the vicinity of two pumping tests at the site are consistent with the K estimates from those tests, providing evidence that the direct-push estimates are of a reasonable magnitude. The results of this field demonstration show that direct-push profiling has the potential to characterize highly heterogeneous aquifers with a speed and resolution that has not previously been possible

Quantitative Analysis of Unconsolidated Coarse Fluvial Sediments from the Boise Hydrogeophysical Research Site: Statistical Analysis of Core and Porosity Data

The qualitative and quantitative analysis of the distribution of hydrologic parameters such as porosity and permeability increases our ability to understand and model ground water flow and contaminant transport in heterogeneous hydrogeologic systems. Core and porosity logs from the Boise Hydrogeophysical Research Site (BHRS), a research well field emplaced in a shallow, unconsolidated, cobble-dominated alluvial aquifer, were analyzed and their statistical characteristics were employed to determine the textural variation within this type of deposit as a whole. The spatial variations of these characteristics within these deposits exhibit strong controls over permeability distributions of the deposit. Core from 12 of the 18 wells at the BHRS were analyzed in detail and divided into sample intervals. Each of these sample intervals underwent a grain-size analysis that was used to determine physical characteristics of the core material and to derive a grain- size based lithotype for each sample. After some simple corrections for artifacts of the drilling process and elimination of intervals consisting of drilling slough, the analyzed core consisted of 953 sample intervals that were classified into one of five lithotypes. Three of these lithotypes contain a large proportion of matrix material, but represent a small proportion of the deposit. These three lithotypes are: sand to fine pebble, floating gravel, and bimodal. The majority of the samples were dominated by cobbles and had gravel to cobble-sized clast-supported framework with interstitial matrix; these two lithotypes are mixed gravel and cobble dominated. The grain-size analysis based lithotypes only take into account the solid-volume fraction of the core; porosity must be added to each sample in order to account for the total volume fraction of the core samples. Porosity logs for the boreholes at the BHRS were derived from neutron logs recorded at the site. Porosity values varied at the site from less than 15% to greater than 40%; however research conducted by Barrash and Clemo (2000, 2002) on the porosity logs show the existence of five hydro stratigraphic units at the BHRS. Principal component analysis and transition probability analysis were carried out on the samples both for the deposit as a whole and for the hydro stratigraphic units. One of these units is almost entirely comprised of sand to fine gravel lithotype and is a continuous sand channel in the western half of the BHRS. Two of the units exhibit low porosity values with low variance and contain only the mixed gravel and cobble dominated lithotypes and show random independent vertical transition structure. The other two units include all five of the lithotypes but are mostly comprised of the cobble-dominated lithotypes, have higher porosity values and higher variance than the two low-porosity low porosity variance units described previously, and have vertical lithotype sequences that are statistically different from random independent and show a Markov vertical transition structure

Significance of Porosity for Stratigraphy and Textural Composition in Subsurface, Coarse Fluvial Deposits: Boise Hydrogeophysical Research Site

Recognition and quantitative characterization of subsurface stratigraphic units in coarse unconsolidated fluvial deposits are difficult because large grain size and the large scale of sedimentary structures make direct interpretation from core difficult or impossible. In this paper, we use porosity data from well logs and grain-size distribution (GSD) data from core to investigate four pebble- and cobble-dominated units that have been identified in porosity logs in deposits at a research well field (Boise Hydrogeophysical Research Site). Lacking direct observation at an appropriate scale, questions about distribution of parameters and textural composition in these units are analyzed with statistical tests. The four pebble- and cobble-dominated “porosity stratigraphic” units may be grouped into two types: (1) Units 1 and 3 have low porosity (mean ∼0.17–0.18) and low porosity variance; and (2) Units 2 and 4 have higher porosity (mean ∼ 0.23–0.24) and higher porosity variance. Based on GSD data, core samples are subdivided into five lithotypes. The five lithotypes occur in different proportions and have different vertical transition probability characteristics in the two types of units: (1) Units 1 and 3 have only framework-gravel–dominated lithotypes and have random vertical transition probability between these two lithotypes; and (2) Units 2 and 4 consist of both framework-gravel–dominated and sand- or matrix-dominated lithotypes and have structured vertical transition probability. The two framework-gravel–dominated lithotypes occur in all four stratigraphic units but have distinctly lower porosity in Units 1 and 3 (i.e., tighter packing) than in Units 2 and 4 (looser packing). Considering the repeated stratigraphic occurrence of (and the statistical significance of differences between) the two types of units, both the individual unit distinctions and the two unit groupings appear to be valid. It is reasonable to interpret that the observed packing differences associated with Units 1 and 3 compared with Units 2 and 4 are related to different sedimentary processes that produce different bedforms or grain fabrics, perhaps under different bedload transport rates