Environ. Sci. Technol. 1994, 28, 1983-1986
Facility-Altered Stable Isotopic Ratios of Power Generation Cooling Wastewater: Opportunity for Tracing Leakages Neil L. Ingraham'
Water Resources Center, Desert Research Institute, University of Nevada System, P.O. Box 19040, Las Vegas, Nevada 89132 R. J. Johnson
Consulting Geologist, 717 6th Street, Boulder City, Nevada 89005 Robert Broadbent
Broadbent and Associates, Inc., 833 Nevada Highway, Suite No. 4,Boulder City, Nevada 89005 Naturally occurring stable isotopes of water (D/H and 180/160 were ) successfully used to demonstrate the lack of relationship between the evaporation pond water of a coal-fired electric generating power facility and an anomalous and substantial increase in the total dissolved solids content of an on-site monitoring well. Water related to the evaporation pond displays various degrees of heavy isotope enrichment from open system free-surface kinetic evaporation along a (6D/6l80)slope of 4.2 originating from a point reflecting the partial evaporation in the cooling tower under near-equilibrium conditions. The stable isotopic ratios of the water as altered by the operation of the facility allow the tracing of potential leakages from the facility. This method we report is considerably less expensive and more definitive than alternate methods of water chemistry and also provided a method of elucidating the phase-change partitioning, mixing, and evaporative losses of different waters in the power facility.
Introduction Increasing regulatory and environmental concerns have required the monitoring of evaporation ponds to ensure their integrity against breaches and subsequent leakage. The process of evaporation tends to increase the TDS (total dissolved solids) and contaminant content of the evaporating water, so contaminants that may be below background levels prior to introduction into the pond may eventually reach levels where the water is considered as a contaminated waste. This situation occurs during the production of electric power where water is used as acooling fluid and the cooling quality is restored by evaporation in a cooling tower. Most of the cooling water is recycled back to the facility, but to ensure quality, portions must be bled off and discharged to an evaporation pond where the water is confined as contaminated waste. The evaporation pond becomes a catch-all for wastewater produced at the power facility, and the quality of the surrounding groundwater must therefore be monitored to ensure the integrity of the pond lining (I). Any TDS increase in local ground water may indicate a breach in the lining of the pond and be of great concern. This situation was recently observed on the site of a coal-fired electric power generating facility in southeast California. The coal-fired power facility is located on an alluvial fan in a closed hydrographic basin in the Mojave Desert ~
* Corresponding author; e-mail address: 0013-936X/94/0928-1983$04.50/0
[email protected].
0 1994 American Chemical Society
and has the capacity to produce between 18 and 20 MW of electricity. The basin is filled with tens to hundreds of feet of quaternary to tertiary alluvial material and is surrounded by mountain ranges between 7000 and 9000 ft above mean sea level. Groundwater flow has been interpreted as generally mimicking the slope direction of the alluvial fans. Water quality varies spatially and temporally within the groundwater system of the basin. Naturally occurring poor quality groundwater is found in the lowest portions of the valley near the playa, while the better quality water occurs beneath the alluvial fans but generally degrades with depth. The power facility is located on a transition zone between good quality groundwater upgradient and poor quality groundwater downgradient. Naturally occurring poor quality groundwater may be drawn toward the facility by changes in the groundwater flow direction resulting from local pumping. Wastewater generated from the facility is managed in a 5-acre lined evaporation pond. The pond is the waste management unit of the facility and is classified as a class I1 surface impoundment. Wastewater in the pond is comprised of bleed-off water from the cooling tower, wastewater from areverse osmosis supply-water treatment system, and wastewater from boiler blowdown. The integrity of the evaporation pond and the quality of local groundwater are monitored by four monitoring wells located on the site. This project was in response to an abrupt increase in TDS (measured in mg/L) detected in monitoring well MW4 as shown in Figure 1. Monitoring wells MW1 and MW2 displayed no increases in TDS above background concentrations during the period of record, while MW3 did show an anomalous increase in TDS to a maximum of near 850 mg/L for a short period. The increase in MW3 did not reach alarming levels and is of no concern here. Water from MW4, however, showed a recent increase in TDS to nearly 5000 mg/L and was of great concern. The increases in MW4 could have resulted from naturally occurring poor quality groundwater, past or present leakage(@ from the evaporation pond (TDS 10 600 mg/L), or washdown surface water (TDS -2 400 mg/L) that may have pooled at the surface around MW4 and leaked down along the well casing. We report here on a definitive method of determining the source of water of elevated TDS observed in MW4, as an alternative to expensive and sometimes inconclusive water chemistry methods. We successfully demonstrated that the ratios of naturally occurring stable isotopes of
-
Environ. Sci. Technol., Voi. 28, No. 11, 1994
1983
5000
m 4 4000 -
2000
1000
/ I
i
I
-
Mw3 Mwl. M w 2
oxygen and hydrogen, by the systematics of stable isotope fractionation, may be used to ascertain whether water from the evaporation pond is causing the observed increase in TDS. Results from this stable isotope study have also helped to understand the phase-change partitioning of the water used in the power facility and have encouraged further research into the use of stable isotopes to determine whether leakage has occurred from other evaporation ponds.
Methods Natural waters always contain dissolved and suspended minerals and organic substances. The nature and concentrations of these impurities may contribute to an understanding of the chemical and biological reactions that have occurred. Thus, studies involving these impurities concentrate on how the lithosphere and the biosphere have affected the water. However, light stable isotopes of hydrogen and oxygen are much more powerful tools for elucidating the history of the water because they are the principle components of water: the hydrogen and oxygen atoms. Light stable isotopes of hydrogen and oxygen are an intrinsic part of the water molecule. Isotopes contain different numbers of neutrons which cause differences in the masses of the nuclides, while the chemical characteristics (which are controlled by valence electrons) remain unaffected. Natural fractionation or separation of the naturally occurring isotopes in water is associated with pathways in the hydrologic cycle, in particular evaporation and condensation. The isotopic compositionof water is generallyunaffected by infiltration, recharge, and groundwater movement. Thus, the stable isotopic signature of evaporation previously occurring on the surface will be retained once the water is in the subsurface and may be determined by isotopic analysis. SampleCollection. Numerous samples were collected for stable oxygen and hydrogen isotopic analysis from all four monitoring wells, two supply wells, the cooling tower and released vapor, outwash water and inflow water to the evaporation pond, sump from beneath the first liner of the double-lined pond and lysimeters collecting water beneath the lower liner of the pond, and the evaporation 1984 Envlron. Sci. Technol., Vol. 28, No. 11. 1994
Table 1. Stable Isotopic Composition, TDS in mg/L, Type of Water, and Date of Collection of Water Sampled for This Study date 1-26-93 1-26-93 1-26-93 1-26-93 1-26-93 1-26-93 1-26-93 12-7-93 12-7-93 1-26-93 1-26-93 1-26-93 2-03-93 2-03-93
sample
TDS (mg/L) 6D (%o) P O (L)
type
SW1 supply water SW2 supply water MW1 monitoring well MW2 monitoring well MW3 monitoring well MW4 monitoring well WD outwash water CT cooling tower TV tower vapor PD pond inflow PW pond water SP sumppump L1 lysimeter no. 1 L3 lysimeter no. 3
550 400 350 320 300-850 to 5000 2400
5100 10 600 9400 9500
-80 -85 -078 -75 -77 -72 -73 -32 -110 -47 -39 -25 -57 -17
-10.2 -10.5 -10 -9.4 -9.7 -8.8 -9.2 -2.8
-13.4 -5.5 -3.1 +0.2 -3.1 +1.7
pond itself. All liquid water samples were collected in 125-mL polysealed glass bottles. Water vapor was collected from inside the cooling tower by a releasing of the vacuum by a stopcock of 10-3 Torr on a 1-L glass vessel, thus trapping the vapor. The vessel was then connected to a laboratory vacuum line, and the atmosphere was pumped through a double U-trap cooled to liquid nitrogen temperatures to condense the water vapor. The water sample was then frozen into capillaries and saved for stable isotopic analysis. The 1-L collection vessel was of sufficient size to provide enough water for both hydrogen and oxygen isotopic analysis. Stable Isotope Analysis. Oxygen extraction from water was performed by converting water to COZ using the guanidine hydrochloride method, which requires only a 10-pL aliquot of sample (2).The resultant COz gas was then analyzed with an isotope ratio mass spectrometer. The hydrogen isotope ratios were determined by using zinc metal to reduce a 10-pL aliquot of the sample to hydrogen gas (31, followed by analysis on a mass spectrometer. Results are reported in terms of variations from a known standard in delta (6) notation 6D Or 6%
%O =
x 1000 where R is the isotope concentration ratio (e.g., l80/l6Oor - Rsbndard/Rsmdard)
DIH). The worldwide standard for both hydrogen and oxygen (Rstandard) is SMOW,an acronym for standard mean ocean water. SMOW has defined 6D and 6l80values of zero, and the variation of a sample's isotopic ratio (R) from SMOW is measured rather than the absolute abundance of the isotope of interest. 6 values are reported in per mil (%o variations from SMOW) and are precise to f l . O % o for hydrogen (6D) and f0.2%0for oxygen ( P O ) . Results Results of stable isotopic analysis are commonly viewed in 6D - 6l80 space. When the stable isotopic compositions of meteoric waters from worldwide locations are plotted, a linear relationship is observed between the 6D and 6l80 values described by 6D = 8(6l80) 10. This linear relationship is referred to as the global meteoric water line (MWL) and was first described by H. Craig ( 4 ) . The results of the stable isotopic analyses of the samples collected for this study are shown in Table 1and plotted in Figure 2 along with the MWL ( 4 ) and a local meteoric water line (LMWL) representing meteoric water local to southern Nevada (5). The stable isotopic compositions of
+
.
n Tower Vapor
-120
'
I
'
I
'
3
'
I
'
I
'
I
.
b
'
all groundwater sampled from the supply wells and monitoring wells (including that from MW4) are similar and plot between -85 and -72% in 6D and between -10.2 and -8.8% in 6 l 8 0 . The water in these wells, as well as the outwash water (WD), appear to be local groundwater and do not show the effects of evaporation. Cooling tower water has a 6D of -32% and a 6l80 of -2.8% and a sample of water vapor released from the cooling tower is most depleted with a 6D of -110%0 and 6l80of -13.4%. Discharge water to the evaporation pond also plots near the LMWL and is more enriched in the ) local groundwater. Other heavy isotopes (D and l80than samples plot to the right of the LMWL and are enriched up to -17 in 6D and +1.7 in P O ,indicating that the waters have experienced kinetic (free-surface) evaporation.
Discussion The anomalously high and increasing TDS in monitoring well MW4 could be explained in various ways. These include contamination by past or present breaches in the evaporation pond liner, leakage of surface washdown water down the well casing of MW4, or naturally occurring poor quality local groundwater. It is unlikely that water collected from MW4 is a mixture of water leakage from the evaporation pond and natural groundwater. The conservative nature of the stable isotopic compositions as reported in 6 units suggests that, by mass balance determinations, the composition observed in MW4 could only include a small (
