Preliminary Report of Field Investigations

Preliminary Report of Field Investigations

Lake Forest Park Water District / Resource Protection Evaluation Project

John Figge, North Seattle College

This project was undertaken as an academic research exercise, conducted by students and faculty at North Seattle College. This report is prepared in an advisory capacity to professional geotechnical consultants who have been contracted for this project, and should not be taken it itself as a professional report. None of the participants in this project are licensed to provide professional advice to the public in this field.

Project Description:

The Lake Forest Park Water District taps both deep and shallow local aquifers to provide water for its community. The deep aquifers are in pre-Vashon sediments, while the shallow aquifer appears to reside in advance outwash deposits (Esperance Sand) of the Vashon Stade. While the recharge area for the deeper aquifers remains somewhat speculative, the recharge area for the shallow aquifer is exposed around Abby View Pond, and in scattered locales to the south. Here, surface exposures of advance outwash sands permit water to infiltrate into the groundwater system.

Above (north of) Abby View Pond, the upper portion of the potential recharge area is mapped as being covered in Vashon Till. Glacial till being relatively impermeable, this would exclude this parcel from being part of the recharge area. This study was undertaken to confirm that this area is, as mapped, surfaced in glacial till.

Research Methodology:

Eight sites were selected from across this area for sampling (see Figure 1). Two were in park settings, while the rest were on road right-of-way corridors. Sampling was conducted using a 4-inch bucket auger, capable of reaching depths of 6 feet. In the majority of cases, this was an inadequate depth to sample the native sediments.

Synopsis:

Preliminary borehole logs are shown in Table 1

While analytical work on the samples is still pending, a few observations can be gained from our field experience

• There is good reason to believe that the western half of the area surveyed is indeed covered in till. While two of our sites (1 and 4) were likely in fill material, sites 2 and 3 appear to be in glacial till. Of equal significance, the presence of shallow ground-water (site 1) and a shallow sub-surface stream (site 4) support the existence of an impermeable surface over this area.

• None of our samples in the eastern half of the area encountered material which was definitively glacial till. In each case, it would appear that we were sampling fill material. All were relatively soft sediments, despite an abundance of rocks. Much of the fill does appear to have been derived from glacial till, which would suggest that it is a local material, but none of our samplings encountered in-situ strata.

The greater amount of fill material encountered in the eastern half may be coincidental, or might reflect different patterns of utility installation. In most ROW locations, we were sampling in close proximity to utility lines, somewhat assuming that they were installed in narrow trenches. Particularly with heavy sewer and water lines, this is not always the practice. None of the locations (aside from Bobcat Park) were settings which appear to have been filled on a broader scale, making it more likely that we were sampling from the disturbed periphery of the utility corridor. In the western half, site 4 was likely in similar material.

The unfortunate fact is that in communities with extensive underground utilities, much of the right-of-way is used for this purpose. Moreover, utility trenches can be pretty deep, particularly for sewer lines. Along the periphery of deeper trenches, sampling to depths of 5-6 feet wouldn’t always be enough to reach undisturbed material. Because the fill material contains rocks of substantial dimensions, attempts at sampling deeper using hand equipment would probably not be successful.

Further Study

Further study for this project will include a more detailed survey of the area to determine if there are surface exposures, construction projects, or other settings where the native surface can be determined. The results of that survey, including the analytical work on sediment samples, will be included in the concluding report for this project.

Figure 1: Study Area

TABLE 1: BOREHOLE LOGS