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Final Hydrogeologic Report
Maplewood Production Wells
PW-11 and PW-17
City of Renton, Washington
January 31, 1990
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INTRODUCTION
This report presents a summary of the installation, testing and analyses of two new water
supply wells located in the Maplewood Golf Course for the City of Renton (Figure 1).
The purpose of our work was to assist the City and RH2 Engineering with the
development of PW-11 and PW-17 for water supply in outlying areas of the Renton water
supply system. Water quality data will be presented under a separate cover.
As originally conceived, the Maplewood well.field was to consist of two wells located
approximately 600 feet apart. Both wells were to be completed during the summer of
1988 in an aquifer encountered and tested in the Maplewood Test Well (GeoEngineers,
1986). An initial well, PW-10, installed in the vicinity of the Maplewood Test well, was
abandoned after it was determined that sand production associated with well development
could not be stabilized. A second well, PW-11, was installed approximately 600 feet
away. The aquifer was encountered in PW-11 at a considerably greater depth and had
much greater production capability. A third well, PW-17 was installed adjacent to PW-
11 to maximize the yield of the aquifer during the summer of 1989. The well installation
and testing has resulted in a revised conceptual model of the site which is presented in
Figure 2. The gravel in the production aquifer may have been deposited by the
recessional outwash from the receding Puget lobe during the most recent (Vashon)
glacial episode. Figure 2 also presents the vertical configuration of the wells.
The role of Pacific Groundwater Group during the course of the project was to:
o Assist RH2 with the logging of the wells;
o Recommend well screen designs;
o Assist with observing screen placement and well development;
o Manage pumping test activity;
o Analyze pumping test data; and
o Prepare this report.
The work was performed and this report prepared, in accordance with generally accepted
hydrogeologic practices at this time and in this area for the exclusive use of RH2
Engineering and their client, the City of Renton, for the specific application to the
project site. No other warranty, expressed or implied is made.
r JP8803 Page2
SUMMARY AND CONCLUSIONS
o Two new production wells, PW-11 and PW-17 and a nested observation well,
OBW-1, were installed in the Maplewood Golf Course area. PW-11 is completed
with 16-inch diameter casing and PW-17 is completed with 20-inch diameter
casing. The two production wells are 50 feet apart. OBW-1 is completed with
two 2-inch, PVC observation wells: one in an upper alluvial aquifer; and one in
the deep production aquifer.
o The recommended pumping rate for PW-11 and PW-17 are 2500 gpm and 1500
gpm, respectively. The maximum designed transmitting capacity of the screens
are 2500 and 2000 gpm for PW-11 and PW-17, respectively.
o The well efficiency of PW-11 is higher than PW-17 resulting in greater well
productivity. The well loss coefficients are estimated at .6 sec2/ft5 and 5 sec2/ft5,
respectively.
o Well interference between the two production wells adds between 30 and 50 feet
rof drawdown in each well depending on pumping rates and duration.
o The estimated transmissivity of the deep production aquifer is between 45,000 and
r60,000 gallons per day per foot. The storage coefficient is estimated at
approximately 5x10-4. The transmissivity of the aquifer at PW-11 may be slightly
higher than at PW-17.
o The drilling and testing has resulted in a revised understanding of the local
hydrogeologic regime. There appears to be two aquifers beneath the Maplewood
area: a "shallow" aquifer within 50 feet of the ground surface; and a "deep"
aquifer which was encountered between -40 and -100 feet elevation at OBW-1 and
-175 and -250 feet elevation at PW-11 and PW-17.
o The water quality from both PW-11 and PW-17 generally meets state drinking
water standards with the exception that two samples collected from PW-11 show
rresults which are higher than drinking water standards. No detectable quantities
of priority pollutants were measured from either well.
o Video logging was completed for each well. The logs indicate that a few
inches of sand is lodged in the bottom of PW-11 and that approximately
4 feet of coarser material (probably pea gravel) is lodged in the bottom
of PW-17.
Page3
JP8803
Drilling and Well Installation
The two new wells were drilled and completed by Armstrong Drilling of Puyallup,
r Washington using the cable tool method. A third well (PW-10) was drilled to deep
aquifer but no well was completed due to development problems. This boring was
converted to a nested observation well and designated OBW-1. The work was completed
during the summer and fall seasons of 1988 and 1989. Geologic logs of all drilling
activity and well construction details are presented in Figures 3 through 5. The following
paragraphs present a chronological description of the drilling and well installation
procedures.
OBW_1 (pW-10) (G1-24783)
Drilling began at well site PW-10 in June, 1988. This was the first well to be drilled
as part of the current project. The aquifer encountered during the installation of the
Maplewood Test Well was the target aquifer for PW-10. Drilling was completed to the
bottom of this zone (encountered from 150 to 205 feet deep) in June, 1988. The well
was logged by Armstrong Drilling, Mark Semrau of RH2 Engineering, and Russell C.
r Prior of Pacific Groundwater Group. Samples of the target aquifer were collected and
returned to Pittsburgh Testing Labs for grain size analyses. Grain size curves are
presented in Appendix A.
A production well screen was installed into the well in July, 1988. Well development
using the surge block technique began immediately and proceeded normally at first.
After several days of development with no decrease in the sand production, surging was
discontinued. Apparently fine sand encountered in the upper portion of the boring was
migrating down to the screen section during surging.
In September, 1988 the production well screen was removed from PW-10 so that the
well could be advanced to the deep aquifer (which had by this time been discovered in
PW-11). Sixteen-inch casing was advanced to a depth of 312 feet. At a depth of 310
feet brown, gravelly silt was encountered. This material was similar to that encountered
at the bottom of PW-11 and is thought to be weathered siltstone (bedrock). Further
drilling at PW-10 was discontinued.
PW-10 was converted to a nested observation well (OBW-1) using two 2-inch schedule
80 PVC pipe. OBW-ls taps the shallow aquifer and OBW-1d taps the deep aquifer.
Monitoring well construction was completed on March 17, 1989.
PW-11 (G1-24781)
Drilling at PW-11 commenced in June, 1988. The deep aquifer was encountered from
280 to 350 feet, much deeper than expected. At PW-11 it is composed of a very well
graded sandy gravel. Samples of this deep aquifer were collected and returned to
Pittsburgh Testing Labs for grain
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aquife Grain
materials and he large colamn
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Appendix A_ Based upon the gain
I JP8803 Page4
of water available for drawdown, it appeared that this aquifer might be very productive.
A production well screen was. installed into the well in July, 1988. Well development
using the surge block technique began immediately and proceeded normally.
Development was completed after 10 days of surging.
PW-17 (G1-25397)
Pumping tests at PW-11 and analytical modeling (Pacific Groundwater Group, 1988)
indicated that the aquifer would support another well drilled near PW-11 (twinned wells).
Drilling at PW-17 was begun in August, 1989. The well was advanced to 355 feet below
ground surface. The upper portion of the well was logged by the driller, John E.
Armstrong and Mark Semrau of RH2 Engineering. The aquifer zone was logged by
Russell C. Prior and Peter N. Schwartzman of Pacific Groundwater Group. Samples
were collected for mechanical analyses which are presented in Appendix A.
The well screen was installed on October 5, 1989. Development began the next day
using the surge block technique. Development continued for approximately 10 days
during which time the sand production decreased to an acceptable level.
Pumping Tests and Analyses
rTwo pumping tests were completed as a part of this project:
1) A step test and a constant-rate test were completed at PW-11 between August 17,
1988 and August 23, 1988. The Maplewood Test Well was used as an observation
well during the constant-rate test.
r 2) Both step-rate and constant-rate testing were completed at PW-17 from October
20-27, 1989. OBW-ls, OBW-ld, and PW-11 were used as observation wells during
the constant rate test.
Table 1 su•nmarizes the aquifer properties which have been estimated from the various
pumping tests. In general, the wells were tested using short-term, step rate and long-
rterm, constant rate techniques.
The purpose of a step-rate test (well test) is to:
o Evaluate well efficiency;
o Set the rate for the constant rate test; and
o Finish development.
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JP8803 Pagel
The drawdown can be calculated for various pumping rates, times, and radial distances.
s = 264 0 * log .3 T t
T r2S
rwhere:
s = drawdown
Q = pumping rate in gallons per minute
T = aquifer transmissivity (assumed 50,000 gpd/ft)
t = pumping time in days
r = radial distance between wells (50 feet)
rS = storage (assumed .0005)
rGeology
A revised conceptual model of the site has been postulated based on the drilling and
testing completed for this project. The uncertainty of the geology beneath the site could
be lessened by more work which is in the planning stages.
rThe occurrence of the shallow aquifer is not in question. It has been encountered in
every boring completed to date on the site. The Parks department is actively using this
aquifer for irrigation of the golf course. This aquifer, however, is not of interest to the
rCity Water Dept. because of its vulnerability to surface activities.
rFigure 2 presents a possible conceptual model of the site in cross-sectional view. In this
model the "deep" aquifer occurs at different elevations beneath the site. This might be
expected for a deltaic deposit. The gravel in the production aquifer could have been
rdeposited in a delta formed at the edge of a submerged Cedar River valley (an arm of
paleo-Lake Washington) during the recession of the Vashon Puget lobe. Vashon
Recessional gravel which blankets the top of the northern upland supports this model.
The inferred direct hydraulic connection between the pumping wells and the various
observation wells also supports this conceptual model.
rWater Quality
Water quality samples were collected from PW-11 and PW-17 at the end of the
respective long term tests. The laboratory reports are presented in Appendix B.
The samples from PW-11 were analyzed by Analytical Technologies, Inc. for volatile
organics, total organic carbon, total organic halides, total coliform, and priority pollutant
metals. Additionally two samples for iron and manganese were collected at early and
late stages of the long term test.
No detectable quantities of volatile organics, total organic halides and coliform were
measured. The discharge had a noticeable odor of hydrogen sulfide during the pumping
test.
CONCEPTUAL HYDROGEOLOGIC CROSS - SECTION
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Pacific Groundwater Group
JP8803 November, 1989
Figure 2