INSPEC25207 - Laserfiche WebLink

Home Browse Search

(Page 21 MINE ID rY OR PROSPECTING ID # M-1978-066 INSPECTION DATE 8!27/02 OBSERVATIONS INSPECTOR'S INITIALS RCO This inspection was performed by the Division as a result of a written complaint received from a citizen regarding possible offsite damage to property adjacent to this 112 permit. The operator's representative was contacted about the inspection, and a time was arranged to meet at the site. The operator was represented by the persons named on page one, who were present throughout the inspection. The complaint letter states that the irrigation pond located on land across the county road from the gravel pit has possibly been adversely affected by excavation activities in the pit, causing the pond to leak. The pond was apparently lined with bentonite in 1977, though it is not known if the pond is older than that, or if subsequent linings have been installed. The pit predates the issuance of this permit by many years, though it was excavated this deep only after Rio Grande County obtained the permit in 1978. The pit was approved to operate as a dry pit, with excavation and stockpiling, but no crushing. Excavation involves bulldozer, loader and trucks. At the time of the inspection, the pond was being filled from the concrete-lined ditch which runs along the west of the pond and the pit. Water was also running in a concrete lined lateral (supplied from the ditch along the west) which extended along the south of the pond. There was evidence of possibly some leakage from the ditches themselves, through the cracks or spaces between the concrete sections, though that was considered insignificant. There were no cracks or misaligned sections observed on the concrete ditches. The pond was nearly full to the visible high water line. The operator's representative indicated that the pond supplied the circle pivot irrigation in the field to the northeast, which was operating at the time of the inspection. It is not known how long the pond had been filled prior to this inspection. There is a cased irrigation well on the southern bank of the pond, which appeared to be functional but not operating at the time of the inspection. The southern edge of the pond surface was measured to be an average of 170 feet (horizontally and directly north) from the location of the seeping highwall. It is not known how deep the pond is, but the surface was estimated to be approximately 2 feet below the elevation of the surrounding grade on the south. Inspection of the northern portion of the pit floor and highwalls reveals the presence of water, although surface material is coarse and well drained. The lower portions of the sloped western highwall was damp on the surface and below the surface. The steeply sloped northern highwall (from surrounding grade down to a depth of approx 12 feet max) was dry. The vertical northern highwall in the "lower" pit (down to a depth of approx 20 feet) was damp below a depth of 17 feet, and exhibited seepage at that 17-foot level. The northwestern pit floor had impounded water, about 300-500 sq ft of surface and less than a foot deep. Most of the surrounding floor was damp, or exhibited alkali indicating former presence of water. Other locations to the east along the northern highwall also were damp at the lower depths. The eastern end of the northern highwall was dry, as were all other highwalls and slopes, and the remainder of the pit floor (to the east and south). It appears very clear that this pit is intercepting groundwater; the moisture cannot be attributed to recent precipitation or surface run-on from adjoining lands. Given the pattern of seepage and wetted surfaces (slopes, highwalls, and floor) vs locations of dry surfaces, it would appear obvious that the source is not an area-wide aquifer, but rather a localized source above the elevation of the pit floor. This leaves several possible sources: the pond, the well, and the concrete ditches, one or more of which should be considered. The pond's bentonite liner may be compromised, the well casing may be cracked, and/or the concrete ditches may be leaking from a seam. The citizen's letter states that the filled pond quickly drains, which points at least partially to a leaky pond liner. Bentonite is used to seal ponds since it absorbs water and swells dramatically when hydrated. It is nearly impermeable to impounded water and retains its moisture for an extended time when exposed to air. However, if bentonite experiences numerous cycles of fairly complete drying, it is known to develop deep cracks, which subsequent hydration and swelling often will not cause to "heal." These deep cracks have been known to extend through the entire thickness of the liner, allowing impounded water to leak and rapidly percolate into the underlying or surrounding soil. It is this scenario which seems to be most likely and plausible to this inspector. It is also possible that if sufficient alkali or other soluble constituent of the subsoil were being dissolved and leached out of the subsoil supporting and surrounding the pond, voids could have developed, which would accelerate the degradation of the structural integrity of the liner. Other possibilities such as a deep burrowing rodent or substantial tree root could have created a tunnel in the liner.