§257.71. Liner design criteria for existing CCR surface impoundments.

(1) Documentation that the groundwater monitoring network meets all the requirements of § 257.91. This must include documentation that the existing network of groundwater monitoring wells is sufficient to ensure detection of any groundwater contamination resulting from the impoundment, based on direction of flow, well location, screening depth and other relevant factors. At a minimum, the documentation must include all of the following:

(i) Map(s) of groundwater monitoring well locations in relation to the CCR unit(s) that depict the elevation of the potentiometric surface and the direction(s) of groundwater flow across the site;

(ii) Well construction diagrams and drilling logs for all groundwater monitoring wells;

(iii) Maps that characterize the direction of groundwater flow accounting for temporal variations; and

(iv) Any other data and analyses the owner or operator of the CCR surface impoundment relied upon when determining the design and location of the groundwater monitoring network.

(2) Documentation that the CCR surface impoundment remains in detection monitoring pursuant to § 257.94 as a precondition for submitting an application. This includes documentation that the groundwater monitoring program meets the requirements of §§ 257.93 and 257.94. Such documentation includes data of constituent concentrations, summarized in table format, at each groundwater monitoring well monitored during each sampling event, and documentation of the most recent statistical tests conducted, analyses of the tests, and the rationale for the methods used in these comparisons. As part of this rationale, the owner or operator of the CCR surface impoundment must provide all data and analyses relied upon to comply with each of the requirements of this part;

(3) Documentation that the unit meets all the location restrictions under §§ 257.60 through 257.64;

(4) The most recent structural stability assessment required at § 257.73(d); and

(5) The most recent safety factor assessment required at § 257.73(e).

(1) Measurements of the hydraulic conductivity in the uppermost aquifer from all monitoring wells associated with the impoundment(s) and discussion of the methods used to obtain these measurements;

(2) Measurements of the variability in subsurface soil characteristics collected from around the perimeter of the CCR surface impoundment to identify regions of substantially higher conductivity;

(3) Documentation that all sampling methods used are in line with recognized and generally accepted practices that can provide data at a spatial resolution necessary to adequately characterize the variability of subsurface conditions that will control contaminant transport;

(4) Explanation of how the specific number and location of samples collected are sufficient to capture subsurface variability if:

(i) Samples are advanced to a depth less than the top of the groundwater table or 20 feet beneath the bottom of the nearest water body, whichever is greater, and/or

(ii) Samples are spaced further apart than 200 feet around the impoundment perimeter;

(5) A narrative description of site geological history; and

(6) Conceptual site models with cross-sectional depictions of the site environmental sequence stratigraphy that include, at a minimum:

(i) The relative location of the impoundment with depth of ponded water noted;

(ii) Monitoring wells with screening depth noted;

(iii) Depiction of the location of other samples used in the development of the model;

(iv) The upper and lower limits of the uppermost aquifer across the site;

(v) The upper and lower limits of the depth to groundwater measured from monitoring wells if the uppermost aquifer is confined; and

(vi) Both the location and geometry of any nearby points of groundwater discharge or recharge (e.g., surface water bodies) with potential to influence groundwater depth and flow measured around the unit.

(1) The location, number, depth, and spacing of samples relied upon is supported by the data collected in paragraph (d)(1)(ii)(A) of this section and is sufficient to capture the variability of saturated hydraulic conductivity for the soil-based liner components and/or naturally occurring soil;

(2) The liquid used to pre-hydrate the samples and measure long-term hydraulic conductivity reflects the pH and major ion composition of the CCR surface impoundment porewater;

(3) That samples intended to represent the hydraulic conductivity of naturally occurring soils (i.e., not mechanically compacted) are handled in a manner that will ensure the macrostructure of the soil is not disturbed during collection, transport, or analysis; and

(4) Any test for hydraulic conductivity relied upon includes, in addition to other relevant termination criteria specified by the method, criteria that equilibrium has been achieved between the inflow and outflow, within acceptable tolerance limits, for both electrical conductivity and pH.

(1) The models relied upon in this paragraph (d)(1)(ii)(C) must be well-established and validated, with documentation that can be made available for public review.

(2) The owner or operator must use the models to demonstrate that, for each constituent in appendix IV of this part, there is no reasonable probability that the peak groundwater concentration that may result from releases to groundwater from the CCR surface impoundment throughout its active life will exceed the groundwater protection standard at the waste boundary.

(3) The demonstration must include the peak groundwater concentrations modeled for all constituents in appendix IV of this part attributed both to the impoundment in isolation and in addition to background.