British Columbia Mine Reclamation Symposium

Performance of sediment control settling ponds, Texada Quarry Mountjoy, K. J. (Keith J.); Diggon, Harold M.; Vickery, B. J. (Brendan J.) 2005

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PERFORMANCE OF SEDIMENT CONTROL SETTLING PONDS, TEXADA QUARRY  Keith J. Mountjoy, P.Geo.1 Harold M. Diggon2 B.J. (Brendan) Vickery3   1Hemmera Envirochem Inc. Suite 250, 1380 Burrard Street Vancouver, BC V6Z 2H3  2Texada Quarrying Ltd. Van Anda, Texada Island, BC V0N 3K0  3Lafarge Canada Inc. Calgary, Alberta T2H 2H5  ABSTRACT  In early 1998, environmental monitoring at the Texada Quarry revealed large amounts of sediment were generated by quarry operations and transported in surface water run-off containing high levels of suspended solids at various locations throughout the quarry.  In order to control sediment transport at the Texada Quarry operations and prevent discharge into the receiving environment, two sediment control settling ponds were designed and constructed: the Main Settling Pond (MSP) and Ancillary Settling Pond (ASP).  The design and construction were completed in 1998.  A monitoring program was implemented under Permit Q-15 to assess the performance of the settling ponds, with effluent criteria set at 75 mg/L TSS.  Monitoring has been conducted on a monthly basis since construction in 1998.  Monthly monitoring data from August 1998 to July 2003 indicate that the MSP effluent was above the 75 mg/L total suspended solids on only three of sixty occasions, despite peak total suspended solids in excess of 10,000 mg/L.  With one exception, the ASP effluent has been consistently below 75 mg/L effluent criteria.  Influent total suspended solids during this same period reach maximum levels of approximately 600 mg/L. Correlation of TDS and turbidity for effluent suggests that field measurement of turbidity could be used to develop a turbidity threshold for determining if the MSP and ASP effluent were below or above the regulated effluent criteria.  This would also for a reduction in analytical costs associated with this monitoring program.  The design, installation, monitoring and annual maintenance of these settling ponds has been successful in mitigating impacts to the receiving environment from the quarry operations.  INTRODUCTION  The Texada Island Quarry is a limestone and aggregate quarry located at Beale Cove on the west side of Texada Island, approximately 130 kilometres northwest of Vancouver, BC.  The quarry is owned by Lafarge Canada Inc. and operated by Texada Quarrying Ltd.  Under previous owners and operators, limestone has been quarried on the property since the 1940s.  Currently, the quarry produces approximately 5.5 M tonnes of limestone and 0.5 M tonnes of aggregate product annually. The main quarry components are the open pit, processing plant and the conveyor and barge/shipping loading systems. DESIGN AND CONSTRUCTION  During a Stage 1 Preliminary Site Investigation of the property in 1997 at the request of Lafarge Canada Inc., surface water run-off was identified flowing into Beale Cove (ENKON, 1997).  The surface water run-off carrying high levels of suspended solids were created from fugitive dust from crushing plant and conveyor operations and from suspended solids in operation wastewater.  As a result, two settling ponds were designed and constructed, based on bench-scale tests, to produce a final effluent containing 25 mg/L total suspended solids (TSS) content (Keystone, 1998).  The design of the settling ponds were based on on-site flow measurements and computer modelling using Environment Canada rainfall data, which showed a 10-year frequency storm event would generate approximately 855 USGPM (194 m3/h).  Applying a safety factor of 1.17, 1,000 USGPM (227 m3/h) total flow was used for design purposes.  During the design phase, it was determined that two parallel ponds, the Main Settling Pond (MSP) and the Ancillary Settling Pond (ASP), would provide a high degree of operation flexibility, especially during periods of maintenance and cleaning.  It was determined that the main settling pond would be designed with a capacity for approximately 80% of the total design flow.  A particle size of 5 microns (µm) was selected as a design particle size to determine the minimum surface area required.  The calculated design area and depth were then used to calculate the expected hydraulic retention time (HRT).  Bench-scale laboratory settling tests were then conducted and indicated a final effluent TSS concentration of 25 mg/L was reached after 6 hours retention without the need for the addition of flocculants.  For the final design for the MSP, a safety factor of 2.1 was applied and for the ASP a safety factor of 3.0 was applied. A summary of design criteria of the settling ponds is provided in Table 1.  Flow to the MSP and ASP was designed to be controlled by a diversion structure with a rectangular weir. The diversion structure consists of two open channels, one 44 inches (112 cm) wide and the other 7 inches (18 cm) wide.  The larger channel allows flow into the MSP and the smaller channel into the ASP. The smaller channel contains a 16 inch (41 cm) high rectangular weir, which allows water to flow to the ASP at total flow rates greater than 800 USGPM.  Slide gates allow for the flow rate through either channel to be adjusted and are capable of diverting the entire flow to either pond, such as during annual cleaning and maintenance.  OPERATION AND MAINTENANCE  The MSP and ASP are in operation 365 days per year, except during shutdown for cleaning and maintenance.  Annual cleaning and maintenance activities for the MSP typically occur between June and September.  Discharge to the MSP is cut-off at the diversion structure and rerouted to the ASP. Accumulated sediment load in the MSP is drainage and dried from approximately June to late August / early September.  In late August / early September a backhoe is used to remove sediments starting from the effluent and working back towards the influent point.  Dried sediments are stockpiled and then removed with a front-end loader.  The sediments are trucked and added to the cement rock stockpile for recycle.  A similar cleaning procedure is used for the ASP as needed however, this has only been required once since installation.  FIELD MONITORING AND ANALYTICAL  Monthly field monitoring has been undertaken at the MSP and ASP since August 1998.  The objectives of data and sample collection and analysis of influent and effluent at the MSP and ASP are to:  • Develop a historical database of the operating conditions of the MSP and ASP; • Confirm compliance with regulatory requirements; and • Identify abnormal conditions or requirements for maintenance and clean-up.  Tasks completed as part of monthly field monitoring include:  • Observation of the influent and effluent points and flow rates; • Observation of water levels within the settling ponds; • Measurement of turbidity and temperature in influent and effluent samples; and • Collection of influent and effluent samples for laboratory analysis of pH, turbidity and TSS.  DATA EVALUATION  The results of field measurements and laboratory analytical results are tabulated monthly and reported annually to the British Columbia Ministry of Energy and Mines.  The TSS analytical results are compared to 75 mg/L TSS as required by Permit Q-15.  Figure 1 compares total suspended solids in influent versus in effluent for the MSP for the period from August 1998 to July 2003.  During this period, the total dissolved solids in MSP influent ranged from 1 to 13,500 mg/L TSS.  The changes in influent TSS concentrations appear to correlated approximately with monthly precipitation, with no or little influent TSS in drier summer months and high influent TSS typically occurring in wetter winter months (i.e. between November and February).  During this same period, the total dissolved solids in MSP effluent were lower than the effluent criteria of 75 mg/L in fifty- seven of sixty months of operation, ranging from 1 to 216 mg/L TSS.  All three exceedances occurred in winter months (November and December), when surface run-off associated with heavy precipitation was highest.  The peak demand in the MSP occurs in early winter (i.e. November or December).  This peak demand appears to be a “flushing event” in response to accumulated “sediment load” that is carried as suspend solids in surface run-off with the first winter rainfall events.  Once this initial flushing has occurred, then the MSP is able to provide a hydraulic retention time adequate for the removal of TSS to below the effluent criteria.  Correlation of TSS with turbidity, for MSP influent (Figure 2) and effluent (Figure 3) were evaluated to determine if turbidity measurements provide a predictable indicator of the TSS.  Overall, the correlation of turbidity with total dissolved solids for both influent (R2 = 0.7229) and effluent (R2 = 0.9674) were good.  For effluent from the MSP, a field-measured turbidity of approximately 75 NTU’s or less, would indicate effluent was below the permitted criteria of 75mg/L TSS. For the ASP, total suspended solids in influent versus in effluent were also compared for the period from August 1998 to July 2003 (Figure 4).  During this period, the total dissolved solids in ASP influent ranged from 1 to 638 mg/L TSS.  The correlation of influent TSS concentrations in the ASP and monthly precipitation on an annual cycle is less apparent than in the MSP.  During this 60 month period evaluated, the total suspended solids in MSP effluent were lower than the effluent criteria of 75 mg/L in fifty-nine of sixty months of operation, ranging from 1 to 117 mg/L TSS. The single exceedance occurred in a winter month (December), when surface run-off due to heavy precipitation was highest and during the first year of operation.  A correlation of turbidity with total suspended solids in both the influent (Figure 5) and effluent (Figure 6) show strong correlations (R2 = 0.8866 for influent and R2 = 0.8118 for effluent), similar to those indicated for the MSP.  For the ASP effluent, a field-measured turbidity of approximately 60 NTU’s or less, would indicate effluent was below the permitted criteria of 75mg/L TSS.  SUMMARY  Monitoring of the Main Settling Pond and Ancillary Settling Pond at the Texada Quarry for the 5 year period from 1998 to 2003, has provided a historical database of operating conditions.  This database confirms that with few exceptions the sediment control settling ponds are performing as designed and meet permit effluent criteria.  Monitoring data collected to-date indicates that turbidity measurements correlate well with total suspended solids concentrations in effluent.  The correlations between turbidity and total suspended solids suggest that turbidity thresholds could be developed to reduce analytical costs associated with the monitoring program.  Monitoring could be conducted by quarry personnel to more actively monitor effluent TSS.  If necessary, the flow to either the Main Settling Pond or Ancillary Settling Pond could be controlled at the diversion structure to consistently meet the effluent criteria.  REFERENCES  ENKON Environmental Limited, 1997, Stage 1 Preliminary Site Investigation of Holnam West Materials Ltd., Texada Quarry, December 1997.  Keystone Environmental, 1998, Sediment Control Management Plan Design, Holnam West Materials Ltd., June 1998  Table 1: Summary of Design Criteria of the Settling Ponds  Variable Unit Main Pond Ancillary Pond Design Flow (Qd) USGPM 800 200   m3/h 182 45 Average Flow (Qavg) USGPM 240 60   m3/h 54 14 Surface Area at Water Level ft2 32,760 7,729   m2 3,060 720 Hydraulic Retention Time at Qd h 13 18 Hydraulic Retention Time at Qavg h 44 59 Overflow Rate at Qd ft/s 0.000054 0.000057   m/h 0.0595 0.0625 Overflow Rate at Qavg ft/s 0.000016 0.000018   m/h 0.0176 0.0194 Length / Width Ratio   2.4 2.2 Length / Depth Ratio   93 22 Scour Velocity ft/min 8.70 9.40   m/h 159 172 Horizontal Velocity at Qd ft/min 0.36 0.11   m/h 6.5 2.0 Horizontal Velocity at Qavg ft/min 0.10 0.03   m/h 1.9 0.6 Reynolds Number at Qd   951 878 Reynolds Number at Qavg   278 263  Note: Designed by Keystone Environmental, 1998.  Figure 1: Main Settling Pond (MSP) 1 100 10000 1000000 Jul- 98 Oct- 98 Jan- 99 Apr- 99 Jul- 99 Oct- 99 Jan- 00 Apr- 00 Jul- 00 Oct- 00 Jan- 01 Apr- 01 Jul- 01 Oct- 01 Jan- 02 Apr- 02 Jul- 02 Oct- 02 Jan- 03 Apr- 03 Jul- 03 Time (months) T o t a l  S u s p e n d  S o i l d s  ( m g / L ) Influent Effluent Discharge Criteria Figure 2: Total Suspended Soilds versus Turbidity Correlation Main Settling Pond Effluent y = 0.815x R2 = 0.9764 0 50 100 150 200 250 0 50 100 150 200 250 Total Suspended Solids (mg/L) T u r b i d i t y  ( N T U ) Figure 3: Ancillary Settling Pond (ASP) 1 100 10000 Jul- 98 Oct- 98 Jan- 99 Apr- 99 Jul- 99 Oct- 99 Jan- 00 Apr- 00 Jul- 00 Oct- 00 Jan- 01 Apr- 01 Jul- 01 Oct- 01 Jan- 02 Apr- 02 Jul- 02 Oct- 02 Jan- 03 Apr- 03 Jul- 03 Time (months) T o t a l  S u s p e n d  S o i l d s  ( m g / L ) Influent Effluent Discharge Criteria Figure 4: Total Suspended Soilds versus Turbidity Correlation Ancillary Settling Pond Effluent y = 0.6068x R2 = 0.8118 0 50 100 150 0 50 100 150 Total Suspended Solids (mg/L) T u r b i d i t y  ( N T U ) 


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