Presentation on theme: "Observations of Water Table Levels in Assumption Creek and at the South Edge of the Seminary Fen Sam Wetterlin 11/1/13."— Presentation transcript:
Observations of Water Table Levels in Assumption Creek and at the South Edge of the Seminary Fen Sam Wetterlin 11/1/13
Overall Precipitation For some context, we provide the above information about precipitation levels in 2012 and 2013. The graph for 2012 only runs to mid-October, but the year ended near-normal, after a very wet period from May to mid-June. 2013 was extremely wet from April through early August. There was no rain for most of August, but cumulative precipitation was still about 30% above normal at the end of August.
Measurement Locations Water table observation tubes were installed at the locations indicated above, and a few additional locations. They consisted of ½” copper pipe driven into the ground anywhere from 5 to 11 feet deep. The “fASL” values shown above are the ground elevation (feet above sea level), taken from a topographical map, and are approximate. The tube at B (lower right) showed the water table to be more than 8 feet deep in July-August, 2012, and was removed. It won’t be discussed further.
PART 1: WATER TABLE LEVELS JUST NORTH OF THE FIELD We will first examine the data from the observation tubes just south of the fen (north edge of the farm field), at P1-P5. As you move from the farm field to the north, there is an elevation drop, and the actual fen generally begins near the lowest elevation. This is typically somewhat north of the actual SNA boundary line. However, at the very west end (near P1 and P2), there is little elevation change between the field and the fen, and the fen extends slightly south of the SNA boundary line. A preview of the results of our measurements: P1, P2 and P5 are at approximately the same elevations as the immediately adjacent fen. P3 and P4 are approximately four feet above the fen level, so we would expect their water table depths to be larger. Surprisingly, though, the water table is generally much deeper below ground at P1, P2 and P5 than it is at P3 and P4, which means that the actual water table elevation (feet above sea level) at P1, P2 and P5 is generally far below that at P3 and P4.
P1 and P2 (West End of Fen/Field) P1 and P2 are located near the southwest corner of the Seminary Fen SNA. At P1 the field and fen are approximately level, but as you move to P2 the field rises and the fen falls, creating a hillside between them. P1 and P2 are located in about two feet of peat overlying rocky/sandy material. The area outlined in brown pools in the spring due to runoff from the fen. (There is little runoff from the field.) The pooled area is about 250 feet long, measured east-west (top-bottom in lower photo). The vegetation immediately around P1 and P2 is mostly dogbane and morning glories. View toward east from Regional Trail
P1 and P2 (West End of Fen/Field) The water table rises significantly in spring, actually creating several inches of pooling at P2, but falls rapidly when precipitation drops off. Note how much deeper the water table is at P1, probably indicating that the deep soil at P1 is much more permeable than at P2. Brown is ground level. Dark red is P1 water table level. Pink is the bottom of the P1 tube. Something happened to obstruct the bottom of the tube, limiting measurement to 5.5 feet rather than 7.5 feet. Blue is the P2 water table level. Light blue is the bottom of the P2 tube. Measurements with depth equal to the tube bottom may mean the water is deeper than that level. P1 P2
P3 and P4 (Center of Fen/Field) P3 and P4 are located about 30 feet outside the fen, south of a fairly complex area. In the spring, the areas marked “Spring Pond” and “Wet” have standing water, which in the Wet areas is an array of water pockets only a few inches deep, but in the Pond area is a continuous pond as much as 1 foot deep. Later in the year the water table in all these areas drops to a depth of one to four feet. The Dry Area has a few areas of standing water in the spring, but is mostly dry and firm; it is slightly elevated above the wetter areas to the south.
P3 and P4 (Center of Fen/Field Red is P3 level; blue is P4 level, both in feet above sea level (left axis). P3 can also be read as feet below ground (right axis). As it turns out, the P4 level below ground matches P3’s quite closely, so there is no separate indication of the P4 level below ground. P3 and P4 are just slightly below field level, about four feet above the adjacent fen level. Water level fluctuates only about three feet. While not proven by this graph, separate data indicates that the water levels at both closely follow the levels in the adjacent fen (unlike P1 and P5), which also vary about three feet, peaking near surface level in late spring/early summer. That variation within the fen is biologically significant, favoring vegetation (such as reed canary grass) that can withstand both high and low water levels. P3 P4
P5 (Near East End of Field) P5 is located at the bottom of the hillside between the field and the fen. The actual fen boundary (as opposed to the legal SNA boundary) is 20-30 feet north of P5. The fen boundary in that area is quite abrupt, being relatively wet on the north side and very dry on the south side of the boundary. The observation tube is approximately 10 feet deep, and the soil was sandy for the entire depth (with considerable rockiness for the first 3 feet.) As we will see, the water table at P5 is remarkably deep considering that its ground elevation is at or below that of the nearby fen.
P5 (Near East End of Field) P5 is in a grassy area. Just beyond it to the north are goldenrod, sunflowers and cup plants, which are in a relatively dry area. Beyond those the ground elevation rises and the wet fen begins. The P5 water table is normally 6 feet or more deep. It rose five feet during the period of rains and snow melt, but quickly subsided after late may. The groundwater level at P5 is remarkably deeper than that in the very nearby fen. This is probably due to the much greater permeability of the soil at P5 as compared to that in the fen, so that south- flowing groundwater from the fen can percolate much faster than it arrives.
PART 2: MEASUREMENTS WITHIN THE FEN We will now examine the water table at points F1-F9. For F1-F5 we have numerous measurements during 2013. For F6-F9 we have one- time measurements used to create a cross-section profile of the water table.
F1 and F2 The water table at F1 is at surface level in the spring and drops in the summer, though it is a bit more resilient than other places we will examine. A piezometer 9’ deep was also placed at F1; its level was within an inch of the water table, indicating no significant upward or downward flow. The water table at F2 is nowhere near the surface, even in spring. It is shown dropping to 3 feet in the summer, but that was the depth of the measuring tube, so it likely was deeper than that.
F3 and F4 The water table at F3 and F4 is at ground level in the spring and then drops significantly, indicating it is very reliant on local precipitation. There is a data gap between 6/25 and 8/25 so the exact trajectory of decline is not shown, but it likely follows that of P3, which is directly south of F4. F4 is at the stake. F3 is circled; it appears to be under a tree but is actually beyond the tree.
Cross-Section at F4 Additional measurements were taken approximately along a north-south line, and used to create a cross-section profile of the ground surface and water table. (F9 is excluded from the cross-section.) The water elevation declines moving north-to-south. Groundwater flow would follow a similar direction, though a steeper gradient might be obtained if the line proceeded southeast from F8, rather than directly south.
F5 View North from F5 Aug 24, 2013 The water level at F5 is below the surface even in spring and drops in the summer. The measurement tube was only 26 inches deep, so the actual level may have fallen further than the chart shows.
Comparisons Among F1-F5 This graph shows that the various locations F1-F5 have a similar pattern of decline in the summer, suggesting that their water levels are more dependent on local precipitation rather than regional groundwater. (All had a measurement limit of 36”, except F5 which was limited to 26”.)
PART 3: ASSUMPTION CREEK WATER TABLE LEVELS In the area of interest, Assumption Creek runs generally west to east, though it flows only for a very short time after heavy rains. The black line at the right side of the aerial photo marks the boundary between the High Terrace property on the left and the Seminary Fen SNA on the right. The Seminary Dam lies on the SNA property near the upper right corner of the photo. Four observation tubes (A and P6-P8) were placed within the creek bed or nearby on the bank. (Location B is not relevant here.)
Creek Locations The four observation tubes are all near the lower elevation of the “valley” that encloses the creek. The 2001 photo at right shows a typical stretch of valley, with a broad sunken area sloping significantly upward on the north and south edges. The photo was taken midway between P6 and P7.
Location A Near the Creek Water Table>11 Feet Deep An observation tube was placed at location “A”, a few feet south of the actual creek bed, elevated about two feet above the creek bed. Water can pool at the culvert outlet for several days after a rain, so the water table measurement was a surprise: For all of July and August, 2012, after a very wet spring, the water table was deeper than the 11 foot depth of the observation tube. Since it did not seem likely that the water table would ever rise much higher, the observation tube was removed.
P6 Near the Creek P6 was not installed until near the end of the heavy rains of May to mid-June, 2012. It is located just south of the actual creek bed, but still likely within the cone of underground water that would be created by infiltration into the creek bed. The water level declined rapidly after installation, going deeper than the seven foot depth of the measuring tube.
P7 In the Creek P7 was the first tube installed and produced data for two summers. It is located directly in the creek bed with white PVC around the top of the actual copper observation tube. It nicely shows the natural seasonal fluctuation. Note that the water table never gets less than three feet deep. This is a key reason the creek rarely reaches the Seminary Dam: With over three feet of unsaturated porous soil beneath it, the creek suffers rapid loss of water as it flows toward the dam.
P8 Near the Creek P8 is located about 25 feet south of the actual creek bed, perhaps a foot above the elevation of the creek bed. This may make it respond somewhat less to creek water infiltration. In addition, being the furthest downstream of the tubes, creek flow reaches it less often and for shorter periods. The water table is very deep at P8 even during the wettest parts of the year.