Message-ID: <7858528.1075851881975.JavaMail.evans@thyme> Date: Mon, 2 Oct 2000 04:15:00 -0700 (PDT) From: george.robinson@enron.com To: larry.campbell@enron.com Subject: Re: NNG Gomez Remediation Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Transfer-Encoding: 7bit X-From: George Robinson X-To: Larry Campbell X-cc: X-bcc: X-Folder: \Larry_Campbell_Nov2001_1\Notes Folders\All documents X-Origin: CAMPBELL-L X-FileName: lcampbe.nsf Larry- Water Disposal: The current rate of water production has been established as a compromise between what we think is most effective for removing PSH and what we think are reasonable disposal costs. Our objective is to flush out PSH that is trapped in dissolution channels and fractures in the aquifer. Dewatering is not an objective because the shallow aquifer is too prolific a producer. In an effort to flush out trapped PSH we pump 200 bbls of groundwater twice weekly (or 1600 bbl per month). In addition we manually remove any accumulated PSH from recovery wells that do not have pumps in them. Pumping 200 bbls per cycle takes about 24 hours and is adequate to create a significant drawdown of the aquifer in the immediate vicinity of the former pit area. The aquifer will recharge within 24 hours, but we don't start another cycle until after about 60 hours in order to limit the number of cycles to two per week. The number of cycles is limited to two per week primarily for cost considerations, but also because our minor permit specifies a maximum disposal rate of 400 bbl per week. If we were to limit the number of cycles to one per week we would drop our PSH removal efficiency by nearly 50%; likewise, if we were to increase the number of cycles to four per week we would increase our removal efficiency by nearly 100%. Note that beyond about four cycles per week, there will be a deminishing advantage to increasing the number of cycles due to the time it takes to draw down the aquifer and the time it takes for the aquifer to recharge. Cycling the pumps twice per week also fits well into our O&M schedule for other sites in the area. We are spending about $3,200 per month for O&M at this site, about $1,800 per month on water disposal and $1,400 for O&M labor. These costs are directly proportional to the number of cycles per week that we operate the pumps. In regard to the effectiveness of this scheme, we have seen some positive indications in both a reduction of accumulated PSH in the former pit area recovery wells and a decrease in dissolved phase contaminants at monitor wells downgradient from the former pit area. An exception to this is the accumulation of PSH from what appears to be another source located west of the former pit area. This issue is being addressed by the installation of additional recovery wells. Another note regarding water disposal, we had briefly discussed the possibility of irrigating produced water rather than hauling it. We temporarily dismissed this idea over concern that arsenic contained in groundwater might accumulate in surface soil as a result of irrigation. We should revisit this idea next spring if it appears that pumping will continue much longer. Monitor Wells: Although the additional wells that we have planned should adequately delineate the dissolved phase plume, I am reluctant to think that these will be the last. The TRC recently rejected closure requests for about a half-dozen ENA sites. In a recent meeting between the TRC and David Ayers (for ENA), the TRC requested that ENA return to the sites and delineate dissolved phase contaminant plumes to MCL concentrations. Although the TRC has not yet made that request for this site, it is certainly a possibility. This is also consistent with recent draft guidance documents prepared by the TRC that specifies plume delineation to MCL concentrations. Currently, arsenic is the only contaminant in groundwater that is not delineated to MCL concentrations, more specifically, downgradient and east of the release area. We will have to wait and see what results we get from the new wells before we have a better idea whether any additional wells may someday be required. Recovery Wells: This is totally dependent on what we find with the additional recovery wells. I suspect that we are going to confirm a separate source of PSH and will require additional wells to adequately remove PSH. Larry Campbell 10/02/2000 09:09 AM To: George Robinson/OTS/Enron@ENRON cc: Subject: NNG Gomez Remediation George, Ive read through your stuff on the Gomez remediation and need some help. Why are we collecting so much groundwater. Im concerned that charges for twice weekly disposal may be perceived as excessive, even though the costs are reasonable. Are we trying to dewater or what? Other than that, I dont mind ensuring the protection of the upstream spring and think we should get busy on the well installations. Do you think this will do it with respect to no more wells needed. Id like to get to that position as quickly as possible.