September 25, 2012 - City of Bothell Properties · HWA Project No. 2007-098-950 2007 098 Gradient...

September 25, 2012 HWA Project No. 2007-098-950

2007 098 Gradient memo 9 25 12.doc 2 HWA GEOSCIENCES INC.

METHODS On August 29 to 31, 2012, HWA conducted a water level and gradient study of the Bothell downtown study area. HWA collected water levels from 67 monitoring wells located between the Sammamish River on the south and NE 188th Street to the north (Figure 1).

The well locations were plotted based on existing base mapping, which included both surveyed and visually mapped locations. Wellhead elevations are also a combination of surveyed data and CAD-provided ground elevations (Table 1).



Table 1 Ground Water Elevation Measurements

Bothell, Washington

Well Screen Interval

(ft bgs) TOC Elevation

(ft MSL)

DTW Ft below

toc

WL Elev (ft MSL)

Used in Study? Notes

BP Station

IW-5 5-15 77.15 10.95 66.2 Y

IW-6 5-15 79.36 9.9 69.46 Y

IW-8 4-9 77.1 5.9 71.2 Y

Former Northshore School District MW-3 5-15 62.29 1.19 61.1 Y

MW-4 6-11 59.47 1.87 57.6 Y

MW-12 3.5-13.5 60 4.67 55.33 Y

MW-13 5-15 57.35 6.12 51.23 Y

MW-15 5-15 63.2 2.57 60.63 Y

MW-16 4.5-9.5 59.92 3.37 56.55 Y

MW-17 5-15 51.92 7.73 44.19 Y

MW-18 5-15 71.46 9.92 61.54 Y

MW-19 5-15 71.78 10.65 61.13 Y

Ultra Custom Care Dry Cleaners

MW-1 5-15 46.9 9.3 37.6 Y

MW-2 3-13 47.18 6.93 40.25 Y

MW-3 3-13 47.02 6.58 40.44 Y

Former Safeway

EB-5 5-20 40.71 6.49 34.22 Y

Bothell Way NE (former SR 527) Geotechnical Borings

BB-2 8-18 38.83 5.4 33.43 Y

BB-3 10-20 42.29 9.13 33.16 Y

BB-7 10-20 59.88 8.79 51.09 Y





(ft MSL)

DTW Ft below

toc

WL Elev (ft MSL)


BB-10 10-20 72.9 12.33 60.57 Y

BI-3 5-10 38.88 4.37 34.51 Y

Bothell Service Center MW-1 5-20 47.4 8.03 38.62 Y

MW-2 5-20 49.3 8.75 39.8 Y

MW-3 5-20 48.41 7.82 39.84 Y

MW-4 10-25 46.1 7.72 37.63 Y

MW-5 10-25 44.69 7.85 36.09 Y

MW-6 10-25 47.62 8.59 38.28 Y

MW-7 10-25 45.97 8.44 36.78 Y

MW-8 45-50 47.77 9.43 37.59 N

MW-9 45-50 50.11 9.71 39.65 N

MW-10 5-25 49.47 6.77 41.95 Y

MW-11 25-33 NA 8.6 NA N

MW-12 25-33 NA 8.9 NA N

Bothell Crossroads/SR 522 Geotechnical Borings

BC-1 25-35 52.81 29.72 23.09 Y BC-2 25-35 49.86 26.41 23.45 Y BC-3 15-25 37.05 14.37 22.68 Y

BC-5 20-30 37.55 8.12 29.43 Y

BC-10 10-20* 59.86 28.77 31.09 Y Casing extended, resurveyed 8/31 BC-11 10-20* 41.52 10.7 30.82 Y Casing extended, resurveyed 8/31 BC-12 10-20 42.17 10.52 31.65 Y

BC-16 11-21 34.78 8.55 26.23 Y

BLMW-5 5-10 34.97 7.27 27.7 Y

BLMW-8 5-15 39.58 8.59 30.99 Y

BPMW-1 10-25* 56.45 27.55 28.9 Y Casing extended, resurveyed 8/31





(ft MSL)

DTW Ft below

toc

WL Elev (ft MSL)


BPMW-2 32-42 31.97 3.79 28.18 Y

HZMW-1 5-15 41.33 7.54 33.79 Y HZMW-4 8-18 40.5 7.23 33.27 Y HZMW-12 10-20 37.34 9.53 27.81 Y

MW-1 5-15 36.13 7.35 28.78 Y

RMW-4 15-25 35.92 9.54 26.38 Y

RMW-5 13-23 34.87 12.75 22.12 Y

RMW-6 15-25 33.8 11.61 22.19 Y

RMW-7 15-25 34.95 17.53 17.42 Y

RMW-8 20-30 39.88 20.43 19.45 Y

RMW-9 20-30 43.7 15.29 28.41 Y

RMW-10 32-42 36.2 13.09 23.11 N

RMW-11 22-32* NA 19.43 NA N Riser installed to extend above preload.

Notes: *- screen interval based on original ground surface. Well has been extended during construction. Ft bgs - feet below ground surface ft MSL - feet above Mean Sea Level TOC - Top of Casing DTW - Depth to Water NA - Elevation not available



Area monitoring wells are screened in multiple water-bearing zones. The majority of the wells are screened in first-encountered ground water. Several wells, most notably selected HWA geotechnical and environmental monitoring wells and monitoring wells completed on the Bothell Service Center property, are completed in deeper water-bearing zones. The ground water gradient map is prepared from shallow ground water elevations (Figure 1). Fifty-one points were used to generate this map.

GEOLOGY

Based on previous studies and published mapping, areas south of SR522 are underlain by fill and recent alluvial sediments associated with the Sammamish river. These soils include sands, silty sands, silts and peat. Areas generally north of SR522 are underlain by fill and glacial soils including outwash and till. These soils include sands, silty sands and silts, although of greater densities than the alluvial soils to the south. Based on the gradient information collected during this and previous studies, the ground water system across the study area is contiguous across the different geologic regimes. Neither area (alluvial or glacial) appears to contain any laterally extensive low permeability layers or aquitards, based on borehole data from across the study area.

FINDINGS

Shallow ground water flow. The general regional ground water flow is to the southeast. This is consistent with numerous previous studies of ground water gradient at individual sites and properties, and is consistent with the conceptual model of flow from higher-elevation areas to the north and northwest of the downtown area and discharge to the Sammamish River.

The ground water gradient through the area is approximately 0.025 feet/foot. The gradient steepens toward the river (up to 0.07 feet/foot), where the increased gradient is visible at the southeast portion of the map near well RMW-7 (Figure 1). Steeper gradients were also noted at the north end of the study area, at the BP Station / Bothell pool building area.

Vertical gradients. Vertical gradients were assessed at three locations where a pair of wells screened in different water-bearing zones were located near each other. One well pair was compared at the Riverside property at the southeast end of the study area, and two pairs were compared at the Bothell Service Center property, north of Bothell Way NE/SR522 (Table 2). The vertical gradient was calculated by dividing the difference in water level elevations by the vertical elevation difference of the well screens (assumed to be the midpoint of each screen). The vertical gradient was found to be downward at the Bothell Service Center property (MW-7 vs. MW-12, and MW-2 vs. MW-9) and upward at the Riverside pair (BC-3 vs. RMW-10). This is consistent with previous data, as well as the regional ground water conceptual model of upland areas receiving recharge, and the Sammamish River acting as a discharge point for ground water from upland areas.



Table 2 Vertical Gradient Pairs

Bothell, Washington

Well Pairs

Screen Interval (ft

bgs)

TOC Elevation (ft

msl) DTW (ft

toc) WL Elev (ft msl)

Gradient (ft/ft)

Gradient Direction

Riverside BC-3 15-25 37.05 14.37 22.68

RMW-10 32-42 36.2 13.09 23.11 0.025 Upward

Bothell Service Center – adjacent to SR 522

MW-7 10-25 45.97 8.44 36.78

MW-12 25-33 45.97 8.9 36.32 -0.035 Downward

Bothell Service Center - 98th Ave NE

MW-2 5-20 49.3 8.75 39.8

MW-9 45-50 50.11 9.71 39.65 -0.0043 Downward Notes: Ft bgs - feet below ground surface ft MSL - feet above Mean Sea Level TOC - Top of Casing DTW - Depth to Water

LIMITATIONS

The conclusions expressed by HWA are based solely on material referenced in this report. Observations were made under the conditions stated. Within the limitations of scope, schedule and budget, HWA attempted to execute these services in accordance with generally accepted professional principles and practices in the area at the time the report was prepared. No warranty, expressed or implied, is made. Experience has shown that subsurface soil and ground water conditions can vary significantly over small distances. HWA's findings and conclusions must not be considered as scientific or engineering certainties, but rather as our professional opinion concerning the significance of the limited data gathered and interpreted during the course of the assessment.

This report has been prepared on behalf of City of Bothell, for the specific application to the subject property. We are not responsible for the impacts of any changes in environmental standards, practices, or regulations subsequent to performance of services. We do not warrant the accuracy of information supplied by others, nor the use of segregated portions of this report.

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September 25, 2012 - City of Bothell Properties · HWA Project No. 2007-098-950 2007 098 Gradient...

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