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September 13, 2000 Daniel Dorgan BY TELEFAX & FIRST-CLASS MAIL Dear Dan: I enclose for your review a September 12, 2000 memorandum from Kelton Barr of Kelton Barr Consulting, Inc. to the Minnehaha Creek Watershed Distinct (MCWD) concerning the potential groundwater impacts from the Highway 55/62 interchange and related construction. Mr. Barr concludes that the proposed southwest loop pond may result in a permanent diversion of one-third or more of the current base flow to Camp Coldwater Springs. In a May 16, 2000 letter to the Minnesota Board
of Water and Soil Resources (BWSR), MCWD President Pam Blixt pledged that
the MCWD would share its data and analyses concerning the Hiawatha Avenue
reconstruction with MnDOT, the Lower Minnesota River Watershed District
(LMRWD) and the Minnesota In an earlier memorandum submitted to BWSR during
the boundary change proceedings for this area (“Summary of Hydrogeologic
Information and Areas of Potential Concern in the Camp Coldwater Spring
Area,” May 9, 2000), Mr. Barr raised concerns that the southwest loop
pond could act to divert groundwater ?Mr. Barr notes that his analysis is based on limited data and that farther examination of the potential diversion is merited. The MCWD is obtaining peer review of Mr. Barr’s conclusions and will advise you and other interested parties when that review is completed. In the meantime, we welcome your assessment of the data in question and trust that you, the DNR and the LMRWD will take appropriate and timely steps to address this potentially significant groundwater impact. Sincerely, Dale Homuth, Department of Natural Resources |
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Kelton Barr ConsultIng, Inc. MEMORANDUM TO: Eric Evenson/Minnehaha CreeK Watershed District FROM: Kelton Barr/Kelton Barr Consulting, Inc. DATE: September 12, 2000 RE: Potential Impacts of Construction and Dewatering
at the Intersection of State Highways 55 and 62 Ames Construction, Inc., a contractor of the Minnesota Department of Transportation (MnD0T), has applied to the Department of Natural Resources on behalf of MnDoT for a temporary dewatering permit for construction at the intersection of State Highways 55 and 62. This has prompted a review of the information collected by MnDoT and its consultant, Short Elliot Hendrickson, Inc. (SEH). This includes portions of the Final Plan documents, boring and monitoring well logs, and the report prepared by SEH (2000). My review of this information reads me to conclude that the discharge rate from Camp Coldwater Spring may be noticeably decreased by both the temporary and permanent dewatering planned for that intersection. My calculations, based on the above information, indicate that the construction dewatering and the stormwater pond in the southwest clover leaf of the interchange, called the S.W. Loop Pond, could divert permanently one-third or more of the flow to Camp Coldwater Spring. This memorandum describes the basis for this conclusion. It should be noted that these are long-standing concerns that were brought forward previously.2 At that time concerns aboutthe towering of the local water table and the reversal of groundwater flow in the intersection vicinity were raised. These potential concerns were also voiced at the kickoff meeting for the SEH investigation now summarized in SEH (2000). The data recently reviewed appear to confirm these concerns. Southwest Loop Pond The stormwater pond receives stormwater runoff from portions of the highways, conveyed to the pond by several storm sewer tines. The outflow elevation Is 241.0 meters above mean sea level (m MSL) into another storm sewer line that ultimately discharges to ponds along the Mississippi River bluff to the southeast. As a point of reference the elevation of Camp Coldwater Spring is 242.05 m MSL. The bottom elevation of the pond is one-half meter lower, identified by nearby MnDoT boring logs as the top of bedrock3. Water table piezometers and monitoring wells indicate that the pond level is more than 5 meters lower than the existing water table (see attached figures). Taken with the subcut drains shown along the roadways surrounding the pond, this would constitute a major, permanent groundwater discharge point, significantly lowering the water table in the vicinity of the intersection. The S.W. Loop Pond also is north of The buried bedrock valley that, according to MnDoT borings along the LRT right-of-way, Is eroded through the Platteville Formation and Glenwood Shale and into the upper portion of the underlying St. Peter Sandstone4. Hydrogeologic Investigations • Geophysical surveys and fracture trace analysis of bedrock fractures • Slug tests and a pumping test using monitoring
wells and piezometers In the intersection vicinity This information is
briefly discussed below. • The groundwater flow direction indicated by the water table configuration generally agrees with the northeast-trending group of fractures, so that the northeast-trending fractures may be a secondary porosity feature conveying groundwater to the northeast. • The northeasterly flow from the intersection area potentially can be Intercepted by the northwestern-trending fracture (spring fracture) that in turn intercepts Camp Coldwater Spring. • One northeast-trending fracture was identified that intersects with the S.W. Loop Pond area6. This wilt be called the pond fracture. Aquifer Testing Results The slug test results from 11 piezometers and wells
had an average hydraulic conductivity of 57 ft/day. This correlates reasonably
well with the pumping test results from PW-1 (49 and 22 ft/day for the
drawdown and recovery tests, respectively). The other two pumping tests
had lower hydraulic conductivity values, down to a minimum of 3.85 ft/day.
The report concluded that the average transmissivity for the overburden
and upper Platteville aquifer was 0.2392 ft2/min (344 ft2lday). Using this average transmissivity and a general hydraulic gradient of 0.012, determined from SEH (2000) Figure 7, yields a groundwater discharge of 2.9E-03 &lmin per-unit-width of aquifer or 2.1 5E.02 gaL/mm-per ft aquifer width. Over a 1,000-foot width of aquifer, a representative portion of the intersection vicinity, in the intersection area, this would yield 21.5 gal/mm. If the transmissivity is representative of the intersection area, the amount of groundwater reaching Camp Coldwater Spring is approximately of this magnitude. Because the flow rates measured by MnDoT at Camp Coldwater Spring range from 77 to 115 gal/mm, this is a noticeable portion of the discharge from Camp Coldwater Spring. This number could be higher, should the subcut drains intercept groundwater flow further to the west of the intersection. The discharge rate of Camp Coldwater Spring was
measured several times during the pumping test. During the test the discharge
rate decreased by approximately 3 gal/mm. The combined pumping rate during
the pumping test was 31.8 gal/mm. From this one may infer that approximately
10 percent of the groundwater pumped during the test was groundwater that
otherwise would have discharged to Camp Coldwater Spring. This interaction
of the pumping with the spring s flow rate would Potential Effects on Camp Coldwater Spring This information has important implications on the quantity of flow from Camp Coldwater Spring, both temporarily during construction and permanently after construction. They are summarized below. • The diversion of groundwater flow to Camp Coldwater Spring is made up of two components: the elimination of the existing groundwater flow from the southwest and the reversal of hydraulic gradients from the northwest-trending spring fracture to the intersection. These are separately summarized below. • Both the construction dewatering and the permanent lowering of the local water table to the S.W. Loop Pond elevation will intercept groundwater flow that heretofore has flowed to the northeast and is part of the Camp Coldwater Spring discharge. This appears to be on the order of 20 to 25 gal/mm. This estimate could be greater if subcut drains along the roadways effectively intercept groundwater west of the intersection. • In addition, the permanent lowering of the water table by the S.W. Loop Pond to an elevation 241.0 (one meter lower than the Camp Coldwater Spring) has the potential to reverse flow through bedrock fractures and the overburden away from Camp Coldwater Spring and toward the S.W. Loop Pond. Comparing these future gradients with the current hydraulic gradients would infer a future groundwater diversion on the order of 10 gal/mm from Camp Coldwater Spring. If the pond fracture were indeed found to be directly, hydraulically connected to both the pond bottom and the spring fracture, the diversion of groundwater from Camp Coldwater Spring could substantially exceed 10 gal/mm. • Taken together, the S.W. Loop Pond has the potential permanently to divert some 35 gallons per minute, or one-third of the discharge, to Camp Coldwater Spring. Should the pond or any subcut drains intersect a permeable bedrock fracture, the proportion of flow diverted from Camp Coldwater Spring could be significantly higher. • There is a range in transmissivities calculated in SEH (2000) from the pumping test: using calculated transmissivities greater or less than the average value would, in turn, increase or lessen the estimated portion of the groundwater diverted from Camp Coldwater Spring flowage. Similarly, changes in the hydraulic gradient or width of intercepted groundwater flow could change the portion diverted. This analysis is based only on information provided
by the references listed above. The calculations stated contain their
inherent accuracies. This memorandum was prepared by Kelton Barr Consulting,
Inc. The discussions, conclusions and recommendations contained in this
report represent our professional opinions based on currently available
information in accordance with currently accepted geologic and hydrogeologic
practices. 1 Sunderman, Allen H. and Craig L. Kurtz (June,
2000) TH 55/62 Interchange Hydro geologic Evaluation: Short Elliot Hendrickson
Inc.; No. A-MNDOT94O6.02. |