Environ. Sci. Technol. 1985, 79, 231-238
currence of an exceedance as a function of this standard; (iii) the Poisson distribution and parameters which describe the number of occurrences of exceedances over a long period, say a year, as a function of the standard; (iv) the effect of emission controls upon the probability of an exceedance; (v) the Poisson distribution of number of exceedances over future periods as a function of the emission control. On the basis of our models we have concluded that extreme pollutant concentrations can be viewed as rare events; thus, it is desirable to describe the number of exceedances in a fixed interval of time by the continuous time, possibly inhomogeneous, Poisson distribution. The time between exceedances is exponentiallydistributed, and the mean number of exceedances in a fixed period is proportional to the product of three quantities: the length of the period, the probability of occurrence of a single exceedance, and the conditional probability (a seasonal factor) that an exceedance occurs within a certain period of the year given that it occurs at all. The seasonal factor can also include the effects of terrrain, meteorology, and month of the year. A linear rollback model based on the principle of mass conservation can be used to compute the reduction in long-run expected pollutant levels given a known reduction in known or average source emissions. Derived parameters of the future distribution of hourly pollutant concentrations then yield a formula for determining the probability that a given number of exceedances will occur in a future period of time. We have also shown how the inverse problems can be solved, i.e., determining the future constant emission control or abatement factor to meet a desired air quality standard. Referees who reviewed this paper prior to its publication were justifiably concerned with the duplication, overlap, and/or differences between the results of this paper and the MPR method formulated by Peterson and Moyers (1).
To aid in a comparison and to summarize results, I have included Table 11. L i t e r a t u r e Cited Peterson, T. W.; Moyers, J. L. Atmos. Enuiron. 1980, 14, 1439-1444. deNevers, N.; Neligan, R. E.; Slater, H. H. In “Air Pollution”, 3rd ed.; Stern, A. C., Ed.; Academic Press: New York, 1977; Vol. 5. Horowitz, J.; Barakat, S. Atmos. Enuiron. 1979,13,811-818. Larsen, R. I. Proc., Int. Clean Air Congr., 1st 1966,6044. Larsen, R. I. “Proceedings: The Third National Conference on Air Pollution”; U.S. Government Printing Office: Washington, DC, 1967, P H S Publication No. 1649, pp 199-204. Larsen, R. I. J. Air Pollut. Control Assoc. 1967,17,823-829. Larsen, R. I. J. Air Pollut. Control Assoc. 1974,24,551-558. Horie, Y.; Overton, J. US.Enuiron. Prot. Agency, Off. Res. Dev., [Rep.] EPA 1974, Chapter 15. Georgopoulos, P. G.; Seinfeld, J. H. Environ. Sci. Technol. 1982,16,40lA. Knuth, W. R.; Giroux, H. D. Meteorology Research Inc., CA, June 27, 1979, Report MRI 78R-1596. National Academy of Sciences EPRI J. 1983, 8 (9), 23. Larsen, R. I. J. Air Pollut. Control Assoc. 1961,11, 71-76. Breiman, L. “Probability and Stochastic Processes”; Houghton Mifflin Co.: Boston, 1969. Cram&, H.; Leadbetter, M. R. “Stationary and Related Stochastic Processes”; Wiley, New York, 1968; Chapter 12. Larsen, R. I. Environmental Protection Agency, North Carolina, 1971. Research Triangle Park, NC, 1971, Report AP-89. Aitchison, J.; Brown, J. A. C. “The Lognormal Distribution”; Cambridge University Press: Cambridge, 1957. Oliver, R. M. Pacific Gas & Electric Company, March 1980, Report 80-1. Oliver, R. M. submitted for publication in J. Oper. Res. SOC. Received for review January 17,1983. Accepted September 4, 1984.
Detailed Model for the Mobility of Arsenic in Lacustrine Sediments Based on Measurements in Lake Ohakuri John Aggett and Glennys A. O’Brlen Chemlstry Department, University of Aukland, Aukland, New Zealand
The mobility of arsenic in sediments in Lake Ohakuri, a hydroelectric storage lake on the Waikato River in New Zealand, has been monitored between 1980 and 1982 and the release of arsenic to the overlying water related to seasonal changes in both lake water and sediment. In shallow areas of the lake the release of arsenic contributes to the continuous seasonal variation in the arsenic concentration in the lake water. In areas that become stratified in summer the arsenic released from the sediments accumulates in the hypolimnion until turnover when it is mixed with epilimnetic water. It has been estimated that the turnover effect results in a temporary increase in the arsenic concentration of between 10 and 20%. Important chemical transformations have been identified, and a model for the system has been compared with previous theoretical models for lacustrine systems. Introduction
In 1972 Ferguson and Gavis (1) published a model for *Until May 1985, address correspondence to this author at the Chemistry Department, Arizona State University, Tempe, AZ 85287. 0013-936X/85/0919-0231$01.50/0
the arsenic cycle in a stratified lake based on theoretical considerations and the results of studies on arsenic in other environments. According to this model accumulation of arsenic in sediments occurs through the formation of arsenious sulfide and ferric arsenate while release to the lake water occurs through “reduction” and the formation of methylated arsenic species. In a subsequent model Wood (2) emphasized the role of the methylated arsenic species in providing a pathway for desorption of arsenic from sediments. Neither of these models was based on any observations on real sediment systems, and the purpose of this paper is to present a detailed model for the mobilization of arsenic in lacustrine sediments based on measurements in Lake Ohakuri for comparison with those postulated by Ferguson and Gavis (1)and Wood (2). The model presented here is the result of a 2-year study in which changes in iron and arsenic in both solid phase and interstitial water of sediments were monitored and related to changes in the overlying water. The monitoring of iron was included in the program as it had previously been shown that iron was involved in the main mechanism for the adsorption of arsenic in surficial sediments (3).
0 1985 American Chemical Soclety
Environ. Sci. Technoi., Vol. 19, No. 3, 1985 231
WAIKATO HYDRO-ELEC TRlC LAKE SYSTEM
Flgure 1. Waikato hydroelectric lake system showing existing and proposed geothermal power stations at Wairakei and Ohaakl, and the areas of the lakes are in kilometers squared.
Table I. Sedimentation in Lake Ohakuri" mean depth of sedimentation since lake formation
Waikato River 26 cm
mean sedimentation rate
Whirinaki arm 43 cm Waikato River 1.5 cm year-' Whirinaki arm 2.5 cm year-' 125 tons
total accumulated arsenic since lake formation mean accumulation rate a
Flgure 2. Study sites on Lake Ohakuri.
8 tons year-'
Data obtained in 1978.
Lake Ohakuri is one of eight hydroelectric lakes on the Waikato River in the North Island of New Zealand. The Waikato River passes through several active geothermal areas about 200 km from its mouth. Effluents from geothermal sources in these areas contain significant amounts of arsenic, the most important single sowce being the Wairakei power station operated by the New Zealand Electricity Department which currently contributes about 65% of the arsenic discharge to the river. Untreated wastewater from the Wairakei power station has been discharged continuously since 1961, and it has been estimated that approximately 190 tons of arsenic enters the river from this source each year (4). Concern over the fate of this arsenic led to a survey of the river system in 1976 (5). Contrary to the results of an earlier survey (6) the results of the 1976 survey indicated that the most important environmental problem appeared to be associated with the accumulation of arsenic in the sediments of eight hydroelectric lakes downstream from the geothermal power station. These lakes are shown diagrammatically in Figure 1. The known geothermal fields at Wairakei, Ohaaki (Broadlands), Waiotapu, and Orakei Korako and all upstream of Lake Ohakuri. The extent to which arsenic has been adsorbed by the sediments was determined through an extensive core sampling program in which 48 cores were obtained from transects at regular intervals across Lake Ohakuri, together with at least six cores from positions in the channels of each of the other lakes. Data for lake Ohakuri are summarized in Table I (5). Although there was noticeable variation in the appearance of the cores, they were generally blackish and gellike in appearance in the upper regions (15-50 cm) and gray and pumicy lower down. Some showed evidence of grayish striations in the predominantly black upper regions, and several had 1-2-cm brown bands at the sediment-water interface. Green striations were frequently observed, most commonly below the surficial brown bands, and, where these bands were absent, in the vicinity of the sedimentwater interface. Loss of weight on drying indicated that the cores contained 80-90% water apart from the near232 Envlron. Sci. Technol., Vol. 19, No. 3, 1985
surface layers where the water content was normally in excess of 90%. Chemical analysis of dried sediment revealed that the solid phase was predominantly siliceous with significant amounts of iron, aluminum, and organic material together with smaller amounts of calcium, manganese, phosphorus, sulfur, and arsenic (3). There was a distinct possibility that these sediments would contain methylated arsenic species if for no other reason than that aquatic weeds in the lake were known to accumulate high levels of arsenic, with analyzed samples commonly containing between 500 and lo00 kg g-l arsenic on a dry weight basis.
Experimental Section Two deepwater sites (I and 11, Figure 2), and two shallow-water sites (111and V, Figure 2) in the Waikato river section of the lake were chosen as study sites. One site in the Whirinaki arm (IV, Figure 2) was also studied. Although site IV was also in deep water (e30m) it was anticipated that behavior in the Whirinaki arm might differ from that in the Waikato River section because the waters in the Whirinaki arm appeared more eutrophic, and water flow from this area into the main lake is restricted by the flow in the main lake which is used to control both power generation and the lake level. Flow through the Ohakuri power station normally varies between 100 and 250 m3 s-l. Temperature and dissolved oxygen measurements were made with a Yellow Springs Model 54 ARC dissolved oxygen meter. Water samples were obtained with a van Dorn bottle. They were filtered through 0.45 pM membranes and analyzed immediately for iron(I1)by using the method of Fadrus and Maly (7). Further filtered sample was preserved by acidification to pH 2 with hydrochloric acid and analyzed for total soluble iron, total arsenic, and arsenic(II1) (8) on return to the laboratory. Sediment samples were obtained with a Jenkins sampler. On retrieval, a sample of the overlying water was taken for iron and arsenic analysis, and the cores were then sectioned at 1-cm intervals in a core extruder. Each section was immediately transferred to a 50-mL centrifuge tube, unoccupied space at the top of the tube was flushed with nitrogen, and the tube was sealed. Interstitial water was separated by centrifugation within a maximum elapsed time of 4 h from the time of sampling. The anion-exchangelatomic absorption spectroscopy procedure of Aggett and Kadwani (9) was used for speciation of arsenic
9
Table 11. Accumulation of Arsenic in Hypolimnion year
As, kg
total vol, lo6 m3
1981 1982
885 563
5.43 4.98
normal [As], normal As, mg mT3 kg 35 35
190 174
AAsa 700 389
Arsenic accumulated by release from sediments.
n\
I
b24 FEB 81
-40
so
100
150
200
250
IAR ez Ltu 21 JAN
2L FEE%
62
SO
100
150
200
I
ARSENIC CONCENTRATION Ipg/LI
Figure 4. Concentration of arsenic in water at site I1 during stratification period 1982.
in interstitital water. As only inorganic species were found in the samples analyzed in this way routine analysis for arsenic(II1) and total arsenic were carried out by the method of Aggett and Aspell (8). Iron in interstitial water was determined according to the procedures used for lake water immediately after separation. The sediment solid phase was dried at 60 "C and analyzed on a dry weight basis after digestion in nitri-ulfuric acid.
Results and Discussion Seasonal changes in temperature and dissolved oxygen as a function of depth in the lake water were very similar at the deepwater sites I and TI in the main lake. Stratification appeared to be limited to those areas where the depth of the lake exceeded 25 m. These constituted
