Todd Tietjen, Eric Wert, Julia Lew and Alan Sims. Southern Nevada Water Authority Las Vegas NV

Transcription

1 Todd Tietjen, Eric Wert, Julia Lew and Alan Sims Southern Nevada Water Authority Las Vegas NV

2 Virgin/Muddy River <2% Inflow Colorado R 97% Inflow Las Vegas Wash <2% Inflow One of the 2 largest reservoirs in the country Primarily supplied by the Colorado River Drinking water supply for Las Vegas, Southern Arizona, and much of Southern California Water supply, flood control, recreation,and hydropower Lake Mead National Recreation Area receives millions of visitors every year Hoover Dam Outflow

3 January 2007 June 2007 January 2008 May 2008

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6 Phosphorus loading limit during growing season Voluntary year round phosphorus removal Voluntary year round (optimized) phosphorus removal Chlorophyll a ug/l 400 Quagga mussels discovered Introduction?

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8 Colorado River/Lake Mead 2 Direct Filtration Plants 900 MGD Capacity/35 GLD Preozonation Ferric Chloride Coagulation Tri Media Filtration Free Chlorine

9 Initial efforts simply focused on cleaning intake structure in the lake Cleaning of intake tunnel during planned shutdown Colonization greater at lake end >100 m 3 of quagga shells in a pile Chemical feed pipeline added Free chlorine used to limit colonization Use of Free Chlorine increased trihalomethane (THM) formation

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11 Seek alternative chemical treatment options for the future Free Chlorine Effective, YES THMs Chloramines Effective? Persistent, low THM Chlorine Dioxide Effective? Produced onsite Permanganate Effective? Pink and yellow water

12 To evaluate alternative oxidants for treatment of the drinking water intake structures Molluscicides, biocides, and copper compounds were not considered Drinking water supply No secondary benefit (disinfection) Free Chlorine, Chlorine Dioxide, Chloramine, and Potassium Permanganate All considered in the past, some advantages and disadvantages of each

13 Treatment Goal: Reduce or prevent settling of veligers Mortality is desired but not required Treatment plant will ensure removal How to assess the impact of oxidants on the benchtop? Mortality difficult to measure Settling cannot be measured in the short term Motility as a measure of oxidant effectiveness May indicate dead veligers Cessation of swimming implies negative impact, hopefully closing of shell and loss of settling/attachment

14 Captured veligers, ~15 m tow of a 63µm plankton net, ~1 veligers/l Each tow was re suspended into 250 ml lake water Individual containers were used for each sampling interval, oxidant combination Oxidants tested: sodium hypochlorite (free chlorine), chloramine, potassium permanganate, chlorine dioxide 4 or 5 concentrations each, based on anticipated residual

15 Oxidants added to individual containers and inverted to mix Samples incubated for 5 minutes 1 ml subsample withdrawn from the bottom of the sample tube and transferred to microscope slide 5 minute additional exposure and acclimation interval Entire 1 ml subsample examined and active and inactive veligers counted

16 Oxidant Residuals Free Chlorine Residual (mg/l) Free Chlorine 1.0 mg/l 0.50 mg/l 0.25 mg/l mg/l Chloramine/Total Chlorine Residual (mg/l) Chloramine 0.50 mg/l 0.25 mg/l mg/l 0.06 mg/l Time (min) Time (min) Chlorine Dioxide Residuals (mg/l) Chlorine Dioxide 0.75 mg/l 0.50 mg/l 0.25 mg/l mg/l Potassium Permanganate Residual (mg/l) Potassium Permanganate 0.40 mg/l 0.20 mg/l 0.15 mg/l 0.10 mg/l 0.05 mg/l Time (min) Time (min)

17 Control Samples

18 Veliger Response

19 No oxidants at inlet Free Chlorine Chloramines

20 Lake Mead and Quagga Invasion lake wide would appear to be complete in 2008 Veliger abundance is our only lake wide measure of quagga population dynamics Veliger numbers generally stable There may be a reduction/shift in veliger abundance from 2 peaks to a single peak period of reproduction Little indication in shifts in the zooplankton community overall Abundance and species composition appear unchanged to date Chlorophyll a concentrations have decreased lake wide Multiple causative changes, decreased P likely most significant Continued variability in lake levels may be a factor in the population dynamics since introduction

21 Oxidants and Quagga Inspection of intake structures soon after the introduction of quagga dictated some action be taken Free chlorine appeared to be effective THM formation increased Search for alternative oxidants to limit colonization Determine an experimental measure of veliger susceptibility Motility Chloramines appear to present the best option for veliger control for SNWA Reduced THM formation Effective in reducing quagga activity at low concentrations Likely indicates settlement prevention Future measurements of veliger activity in raw water planned Future evaluation of effectiveness at pilot plant scale planned

22 Lake Mead Limnology Todd Tietjen Alan Sims Treatment Research and Design Eric Wert Julia Lew