The PVC mat penetration device, Well B1, is located at Station B1 just outside the open water ponds restoration site at Na Pohaku o Hauwahine.
- The following legend applies to the graph above:
- Vertical lines "I" and "II" mark changes in the water level measurement device: before I - two measurements of water or slurry thickness made at this station in April and June during augering and penetration device installation; I - first measuring tape device used (3 measurements); II - final tape device (see photo at right) with modified weight used.
The following are all derived from measurements made with the weighted drop line. The blue circles represent measured height of water above "drop-line" bottom. The green line represents "top of mat" calculated as tape reading from solid bottom to pipe lip, minus pipe extension above mat, plus a correction factor (+0.09 ft) to account for mat depression when making the measurement. The brown line represents "bottom of mat" calculated as "top of mat" minus a mat thickness value. The small blue squares show the depth to solid bottom measured with the weighted line just prior to each leveling survey measurement. These depths are calculated by subtracting the drop line measurement to water level in the well from the staff gage water level reading (corrected) the same day, on the assumption that the two are identical. Vertical lines representing the PVC casing are shown at selected points in time for reference, plotted from the measurement from solid bottom to upper lip of pipe and pipe length. The casing is attached (and moves with) the marsh "surface" (green line).
The following plots are from measurements made at the the water level staff gage or by stadial survey from the staff gage. The fine dashed line is the water level recorded at Na Pohaku o Hauwahine, corrected to a USGS gage located elsewhere in the marsh. All other well measurements are corrected by the same factor as appropriate, although a method of matching the water level at the well and the "marsh" water level is still being developed. The maroon points represent actual elevation measurements of the solid bottom from standard leveling survey techniques using the elevation of the WL gage as the "back sight." Water level in the well read on the survey rod is compared with water level measured with the drop line, and these show relatively good agreement at this location (range of 6 measurements: 0.00 to -0.09 ft where negative values imply the rod was pushed deeper into the mud than the drop line). THe average difference between the drop line depth and the staff depth is the thickness of the soft muck layer on the bottom (see table at right).
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Sta. GPS: Date installed: Pipe length: Ext.above mat: Mat thickness: Under mat: Soft muck layer: Clay layer at |
June 2002 2.66 ft. 0.90 ft. 1.75 ft. (3) water ~1 ft deep 0.03 ft (11) thick +0.69 ft. (12) elev. |
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(n) = number of independent measurements contributing to mean value shown | |
Reasonably good agreement is shown between the water level measured in Well B1 (blue line) and water level in the pond (fine dashed line) located about 10 m away, at least after event "I." It is felt that these two measurements should always agree, although one is made as a reading on a fixed staff gage and the other is a depth to "solid" bottom using a weighted drop line. Presently not known is whether disagreement between these measurements reflects some real phenomenon or is simply the result of measurement error. One source is the lower frequency that well measurements are made compared with staff gage readings. Because disagreement over time appears to be small and not systematic, a "best-fit" method may have the most utility. It seems that the solid bottom is at a fixed elevation at this location.
Well B1 fits the "Floating Mat" Model of the marsh surface. The vegetation mat is clearly floating on a layer of water. This layer is typically 30 to 36 inches thick in this area, and about half or a little less of the water thickness saturates the vegetation mat itself; that is, the 15 to 16 inch thick mat is partly below the water surface. The surface of the mat, however, remains a constant distance above the water surface, indicating that this organic layer or peat is floating unconstrained on water. Note also that changes in water level in the well are not exaggerated relative to changes in water level in the nearby open pond, further indication that water level measurements at Well B1 are being made on a covered lake and not in an aquifer.
The results of cores (B1-1 and B1-2) made with a 4-foot long 0.5-inch diameter glass tube are shown on the right. The core locations are within 3 m of the well. All field measurements were related to water level at the time. The thickness of the floating mat (upper dark layers) was measured separately after a hole was made through it for insertion of the glass tube. This approach was necessary in order to obtain an accurate core (minimal compression) of the liquid layers beneath the mat. The top of the mat is set as the estimated height the measured mat thickness would sit using the approximate mat density (from the well site) at the time, for the reason that the mat is depressed an unknown amount while taking measurements and its exact relationship to the deeper layers is not known. The water layer is shown in white (around 1.6 elevation in B1-1 and -2), with slurry increasing in density down to a dense, clay layer (bottom layer in profile) that was only penetrated far enough to hold the core in the tube for measurements. A "V" marks the depth of penetration of the weighted drop line in the hole. Note that this height above the clay layer in core B1-2 is small, like the long-term average soft muck thickness of 0.04 ft; but is much greater in core B1-1.
The 2002 dry season change in water level represented by measurements made between July 31 and October 14, 2002 averaged -0.008 ft/day (-2.6 mm/day). This dry season decline matches that calculated for the pond water level (-0.009 ft/day or -2.8 mm/day) between the same dates.
