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Distinguishing Outewash, Ablation Till and Basal Till

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Distinguishing Outewash, Ablation Till and Basal Till DISTINGUISHING OUTWASH, ABLATION TILL, AND BASAL TILL WITHIN THE SRSNE SITE POTENTIAL OVERBURDEN NAPL ZONE Purpose and Background The purpose of distinguishing between outwash, ablation till, and basal till is to help understand the migration pathways and general distribution of NAPL in the overburden at the SRSNE Site. NAPLs at the site have densities that range from less dense than water (LNAPL) to denser than water (DNAPL). The LNAPL density has not been measured. The DNAPL densities have been measured as between 1.1 and 1.2 g/ml. The NAPLs all have viscosity similar to that of water, and low interfacial tension. The outwash contains stratification that would be expected to promote lateral spreading of NAPLs, but it is not believed to contain laterally extensive capillary barriers that would preclude downward movement of dense NAPLs (DNAPLs). In addition, outwash typically contains isolated layers, lenses, and "shoestrings" of well-sorted, sand and or gravel where NAPL may have preferentially migrated. It is believed that relatively coarse-grained, linear features, where present, are relict stream channels, generally oriented north-south, parallel to the Quinnipiac River valley. Ablation till contains a significant component of fines (typically silt but also occasional clay), and it can be significantly denser than outwash. Typically outwash has split-spoon blow counts of <10 per 6 inches; ablation till commonly has blow counts >30 per 6 inches. According to TtNUS, till with noteworthy layering or stratification is considered ablation till. BBL interprets that ablation till is generally an effective capillary barrier that would resist or prevent downward NAPL movement (except where compromised by drilling). TtNUS believes that ablation till is not as effective as basal till in impeding downward DNAPL migration because of the presence of sub-horizontal layering in ablation till. Basal till is similar to ablation till, except that (as TtNUS believes) basal till does not contain noteworthy layering or stratification. BBL and TtNUS agree that, where present and not compromised by drilling, the basal till would be expected to behave as an effective capillary barrier to downward DNAPL movement. Assumptions Assumption #1: Where present, basal till is a layer of dense, poorly-sorted soil directly overlying bedrock. Assumption #2: Where present, ablation till is dense, poorly-sorted soil containing noteworthy horizontal layering, which directly overlies either basal till or bedrock. Assumption #3: The less dense, native material above ablation till is considered outwash for the purpose of this classification, although it may also include lenses of fine-grained and/or dense soil. Classification Procedures Step 1. Find the deepest soil interval with <10 blows per 6 inches. That soil interval and all of the overlying native material will be classified as OUTWASH regardless of grain-size distribution. Step 2. Below the interval identified in Step 1, find the next deepest soil interval with blow count >=30 per 6 inches that also contains either silt and/or clay as a major component or prominent sub-component designated by "and" or "some" or adverb-form modifier (e.g., Gravelly Silty Sand; Clayey Sand; etc.). The top of that soil interval will be classified as the top of TILL (either ablation or basal). (All overlying material above the interpreted top of till will be classified as OUTWASH for the purpose of this classification.) Step 3. Starting at the top of till depth interval identified in Step 2 and going downward, identify the deepest soil interval that has a soil description containing the term "layering", "stratification", "bedding", "lamination" or other term connoting sub-horizontal depositional fabric. That soil interval and all other till intervals up to the top of till identified in Step 2 will be considered ABLATION TILL. All deeper soil intervals will be considered BASAL TILL down to the top of bedrock. General Note: The geologic conditions at each boring location will be determined based solely on the information obtained at that specific location. Additional Considerations The terms "angular gravel" and "sub-angular gravel" generally are considered strong indicators of till (either ablation or basal), particularly when the sample also contains either silt and/or clay as a major component or prominent sub-component designated by "and" or "some". APPLICATION OF THIS CLASSIFICATION SYSTEM EXAMPLES FROM EXISTING BORING LOGS B-1: Deepest blow count <10 at 6.5'. Next interval with blows>= 30 at 8', described as "GRAVELLY SILTY SAND, medium to coarse sand, silt, and trace of gravel (SM)". Top of Till at 8'. Outwash above 8.0'. "Layering" described to depth of deepest soil sample. Thus, no basal till. Ablation till 8.0 to top of rock at 13.5 or 14 ft. Notes: • A B-1t , BBL previously interpreted top of till at 7.5', similar to 8.0'. • HNUS previously interpreted top of basal till at approx. 10', although the next two soil samples indicated "layering". (Change in interpretation.) B-4: Deepest blow count <10 at 11.5'. Next interval with blows>= 30 at 13.5', described as "SILTY SAND, SANDY SILT, layered fine to medium sand with silt, some fine angular gravel (SM) fine sand and wilt (SP-SM)". Top of Till at 13.5'. Outwash above 13.5'. "Layering" described in sample from 13 to 15. Ablation till 13.5' to 15'. No "layering" in next (deepest) soil sample. Thus, basal till from 15' to top of rock at approx. 16'. Notes: • At B-4, BBL previously interpreted top of till at 13', similar to13.5'. • HNUS previously interpreted no basal till at B-4. Change in interpretation. The last split- spoon was driven from 15 to 16.3 feet, with a 10-inch recovery of soil described similar to other till samples at the site, and a 1-inch chuck of gravel, presumably at the bottom. This sample would be reinterpreted as indicating approximately one foot of basal till. MW-123C: Deepest blow count <10 at 11.0'. Next interval with blows>= 30 at 16', described as "GRAVELLY SILTY SAND, layered fine to medium sand and silt with some fine to coarse gravel (SM, SM-ML)". Top of Till at 16'. Outwash above 16'. "Layers" described in sample from 18 to 20, but no deeper soil samples. Ablation till 16' to 20'. Basal till from 20' to top of rock at 32'. Notes: Very good correlation with previous interpretations. • At MW-123C, BBL previously interpreted top of till at 16', same as top of ablation till. • HNUS previously interpreted top of basal till at 20'; this is confirmed. MW-125C: Deepest blow count <10 at 24.5'. Next interval with blows>= 30 at 25', described as "SILTY SAND, fine sand and silt with occasional coarse gravel (SM)". Top of Till at 25'. Outwash above 25'. "Layering" described in the sample from 26-28', but no deeper soil sample. Ablation till from 25' to 28'. Basal till from 28' to top of rock at approx. 35'. Notes: • At MW-125C, BBL previously interpreted top of till at 25', same as new top of ablation till. HNUS previously interpreted no basal till at MW-125C. Change in interpretation. Since no "layering" was noted in the soil descriptions below 28', basal till would be re-interpreted as being present from 28' to top of rock. P-5B: Deepest blow count <10 at 30'. Next interval with blows>= 30 at 31.5', described as "No recovery", but next sample described as "GRAVELLY SILTY SAND, sand with silt and gravel (SM)". Top of Till at 31.5'. Outwash above 31.5'. "Layering" described in last soil sample from 36' to 37.5'. Top of rock appears to be at approx. 38'. Ablation till from 31.5' to top of rock at approx. 38'. No basal till. Notes: • At MW-125C, BBL previously interpreted top of till at 28'; the new interpretation would be 31.5'. Change in interpretation. • HNUS previously interpreted no basal till at MW-125C; this is confirmed. SB-701: Deepest blow count <10 at 10.5'. Next interval with blows>= 30 at 13.0', described as "Red SILT and very fine SAND, some gray gravel, hard". Top of Till at 13.0'. Outwash above 13.0'. No "layering" described in any samples from there down. Top of rock not encountered. Basal till from 13.0' to top of rock. Notes: • SB-701, BBL previously interpreted top of till at 13.0'; this is confirmed. • HNUS did not render an interpretation of the top of till units at this boring, which post-dated HNUS's investigative work. .
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