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STIG PROJECT -- EVALUATION OF WATER FOR INJECTION -- 28 March 1994 15 ;600 gpm going to WSWPCF. When silica concentrations are not favorable, all the discharge from the waste receiver tank must be directed to WSWPCF. This section describes how silica concentrations may be monitored to control acceptable splitting of the waste stream. Timing Aspects Less than 5 mg/kg separate the average concentration (59.5 mg/kg) from the critical concentration(64.5 mg/kg).Figure 2A shows that silica concentrations can shift at least as quickly as 1 mg/kg per hour for a few tens of hours. When monitoring silica concentrations, a component of time will be involved with obtaining an analytical result for water currently in the process stream. In practice, a trend cannot be identified with fewer than three sequential samples, and verification could require a fourth and plausibly a fifth sample.Subsequent increments of time will be required to review the data in order to make a decision about changing the split of discharged waste. Once the well is closed, stagnant, supersaturated fluid could deposit minerals in sensitive porosity near the wellbore. The resting time after closure will favor deposition of a tiny increment of mineral as the chemical aspects approach equilibrium. There may be other practical altenatives20, but over the life of the project, many occasions of closure, each with an individually tiny effect, could have a significant cumulative effect. Accordingly a good effort should be made to respond quickly enough to a rising silica concentration that even these small increments of deposition are avoided. Of course, responding too soon carries a statistical risk that a projected rise in silica concentration does not actually reach the critical level, resulting in loss of advantage that injection is intended to give. Too slow a response risks mineral deposition in rock porosity near the wellbore. Monitoring and Data Review The description above appears to require a capability to respond within a small number of hours. Two, or so, could be a reasonable target. This could be met by continuous, or semi- continuous analysis of silica in selected parts of the in-plant flow system. Two silica monitoring locations are indicated in Figure 1. Automated sampling frequency of three per hour, or faster, would be satisfactory. An automated method of reviewing results should be considered because substantial periods of time will pass in which no symptom of risk would be apparent. The automated data review system should respond both to near-critical concentrations and to high rates of change of concentration. Location of silica monitor #1 is indicated (Figure 1) on the discharge of the ultrafilter. This is considered superior to upstream alternatives for analytical reasons.Silica in WSWPCF water may not be entirely monomeric, but it is the monomeric form that is of concern with calculated saturations. Ultrafiltration will probably remove most of the extraneous varieties, providing for a more reliable analytical result from the sampler. Locating the sampler at an alternative,downstream location,such as the RO waste streams, reduce the practical response time for the system. Also, a concentration measured there must be compensated by the current RO concentration factor before it could be reliably applied to other units.Associated error or uncertainty would propagate unfavorably in the arithmetic of assessment. To use the monitoring results,combine them with a multiplier(m*) derived from equation DON MICHELS ASSOCIATES -Missoula,Montana USA