Dissolved oxygen modeling of the Blackstone River (northeastern United States)

Dissolved oxygen modeling of the Blackstone River (northeastern United States)

PII: S0043-1354(98)00004-9 Wat. Res. Vol. 32, No. 8, pp. 2400±2412, 1998 # 1998 Elsevier Science Ltd. All rights reserved Printed in Great Britain 00...

643KB Sizes 2 Downloads 110 Views

PII: S0043-1354(98)00004-9

Wat. Res. Vol. 32, No. 8, pp. 2400±2412, 1998 # 1998 Elsevier Science Ltd. All rights reserved Printed in Great Britain 0043-1354/98 $19.00 + 0.00

DISSOLVED OXYGEN MODELING OF THE BLACKSTONE RIVER (NORTHEASTERN UNITED STATES) RAJAT R. CHAUDHURY1, JOSE A. H. SOBRINHO2*, RAYMOND M. WRIGHT3 and MAKAM SREENIVAS4 1 National Institute of Ocean Technology, Madras, India; 2Technology Planning and Management Corporation, Mill Wharf Plaza, Suite 208, Scituate, MA 02066, U.S.A.; 3203 Bliss Hall, Department of Civil and Environmental Engineering, University of Rhode Island, Kingston, RI 02881, U.S.A. and 4 Pare Engineering Corporation, 8 Blackstone Valley Pl., Lincoln, RI 02865, U.S.A.

(First received February 1997; accepted in revised form December 1997) AbstractÐA dissolved oxygen model, QUAL2E, has been calibrated and validated to the Blackstone River, using the data collected in 1991 for the Blackstone River Initiative Ð Dry Weather Study. Physical representation of the river is accomplished using the data from previous studies. Flow pro®les have been developed using the average daily ¯ows of the three permanent United States Geological Survey (USGS) gaging stations and ®ve wastewater treatment plants on the Blackstone River. These ¯ow pro®les have been veri®ed by the success of the model to simulate a conservative parameter, chloride and independent ¯ow measurements. The calibration and validation of the model for dissolved oxygen involved a stepwise approach to simulate each source and sink of dissolved oxygen. This has been accomplished by progressively de®ning atmospheric reaeration, oxygen depletion due to carbonaceous biochemical oxidation demand, nitri®cation, sediment oxygen demand and algal photosynthesis and respiration. The success of the model can also be judged by the ability of the model to represent the system under di€erent conditions. During additional validation with data sets from 1980, the model was successful in predicting conditions prior to a previous waste load allocation for the largest point source. Statistical tests of the model results and the observed data have been performed for the 1991 surveys with a median relative error of approximately 10%. A brief assessment of the 7Q10 ¯ows indicates violations of the dissolved oxygen criteria of 5.0 mg/l occurring downstream of the Woonsocket wastewater treatment plant. Minor violations occur in Massachusetts upstream of four impoundments. # 1998 Elsevier Science Ltd. All rights reserved Key wordsÐdissolved oxygen, modeling, QUAL2E model, carbonaceous and nitrogenous biochemical demand, reaeration, algal productivity, Blackstone River initiative

b1= b2= N1= N2=

NOMENCLATURE u= stream velocity (m/s) x= stream distance (m) Q= stream ¯ow (m3/s) D= stream depth (m) q= incremental ¯ow factor (m3/km2-s) a, b, c, d= empirical parameters O= concentration of dissolved oxygen (mg/l) O*= saturation concentration of dissolved oxygen (mg/l) rate of oxygen production per unit of algal a3= photosynthesis (mg-O/mg-A) a4= rate of oxygen uptake per unit of algal respired (mgO/mg-A) rate of oxygen uptake per unit of ammonia nitrogen a5= oxidation (mg-O/mg-N) a6= rate of oxygen uptake per unit of nitrite nitrogen oxidation (mg-O/mg-N) A= algal biomass concentration (mg-A/l) K d= carbonaceous BOD deoxygenation rate (dayÿ1) L= ultimate concentration of carbonaceous BOD (mg/l) sediment oxygen demand (g-O/m2-day) K4= m= algal growth rate (dayÿ1) r= algal respiration rate (dayÿ1) s1= local settling rate (m/day)

ammonia oxidation rate coecient (dayÿ1) nitrite oxidation rate coecient (dayÿ1) ammonia nitrogen concentration (mg-N/l) nitrite nitrogen concentration (mg-N/l)

INTRODUCTION

*Author to whom all correspondence should be addressed. [Tel.: +1-781-5443189; Fax: +1-781-5443086, E-mail: [email protected]].

The Blackstone River is a relatively small river system in Northeastern United States in south central Massachusetts (MA) and northeastern Rhode Island (RI). This 76.8 km (48 mile) long river originates in Worcester, MA and ¯ows south to Pawtucket, RI where it discharges into the Seekonk River and ultimately Narragansett Bay (Fig. 1). The Blackstone River has had a signi®cant role in the industrialization of the northeastern region of the United States, and presently an equally signi®cant role in the environmental health and future use of Narragansett Bay. Based upon previous research, the Blackstone River is a major source of pollutants to Narragansett Bay (Wright et al., 1991). As a result, Narragansett Bay resources of ®shing, shell®shing, tourism, and recreation are being impacted by pollution from the river.

2400

Dissolved oxygen modeling

2401

Fig. 1. The Blackstone River watershed and water quality stations.

In recognition of the primary importance of the Blackstone River to the future of Narragansett Bay, the Blackstone River Initiative (BRI) was organized. The initiative, coordinated by the University of Rhode Island (URI) with the United States Environmental Protection Agency (USEPA), the Massachusetts Department of Environmental Protection (MADEP) and the Rhode Island Department of Environmental Management (RIDEM), is a multi-phased basin-wide assessment of the river, tributaries, and discharges under both dry (Phase I) and wet conditions (Phase II). The BRI was established to characterize the current water quality, document the changes and identify and quantify the existing water quality problems in the Blackstone River watershed.

From a historical perspective, although water quality in the Blackstone River has been improved dramatically over the last 20 years as a result of requirements under the Clean Water Act (CWA), several major water quality issues remain. One of these issues is with regards to dissolved oxygen (DO). The dissolved oxygen and nutrients study in the Blackstone River included dry weather data collection and analysis and the calibration and validation of the QUAL2E model. The dry weather program (Phase I) consisted of three 48 h surveys performed in 1991. Twenty three water quality stations were selected for analysis within the Blackstone watershed which encompasses an area of 1230 km2 (480 mi2).

2402

Rajat R. Chaudhury et al.

This article details the application of the data in the calibration and validation of the dissolved oxygen model QUAL2E for the Blackstone River. The model aids in establishing the major factors governing water quality in the river and is a tool that can be used in developing waste load allocations (WLAs).

BACKGROUND

Water quality sampling Water quality sampling stations. For the dry weather surveys there were a total of 23 stations. The stations location map is provided in Fig. 1. These included ®fteen locations along the main stem of the river, six near the mouth of the major tributaries and the two largest point sources (Hartman, 1992). Dry weather ®eld and sampling program. The dry weather program consisted of three 48 h surveys: July 10±11, 1991 for Survey 1, August 14±15, 1991 for Survey 2 and October 2±3, 1991 for Survey 3. Grab samples for DO were collected every six hours over the 48 h period by the USEPA and MADEP ®eld personnel. Temperature, pH, and conductivity readings were taken in the ®eld concurrently with the DO samples. All ®eld measurements were performed using portable meters. For the instream sampling, four sets of discrete grab samples were collected once every six hours over the ®rst 24-h period for each of the three surveys. Instream samples were measured for 5-day biochemical oxygen demand-BOD5 (DO probe), chloride (chloride probe), ammonia (Standard Methods (STM, 1989), Method 4500NH3/D), nitrate and phosphorus (STM 1989, Methods 4500NOÿ 3 /E and 4500-P/E), organic nitrogen (STM, 1989, Method 4500N/B) and chlorophyll a (STM, 1989, Method 10200/H). Samples were collected using te¯on buckets and pre-cleaned plastic bottles. Samples were stored in ice for transport. All analyses were conducted by the Environmental Engineering Laboratory at the URI. Hourly composited samples were collected at the point sources each day for 5 days prior to the survey dates and analyses included all of the above parameters. Sediment oxygen demand (SOD) rates were measured by the USEPA personnel (Biological Section, Lexington, MA). The SOD measurements were performed utilizing a specially built apparatus that con®nes a measurable volume of undisturbed sediment and overlying water in a core cylinder and measures the depletion of DO over time. QUAL2E model theory QUAL2E is a steady state stream water quality model that primarily simulates DO and water quality parameters that in¯uence DO concentrations. It assumes that the major transport mechanisms are

signi®cant only in the direction of ¯ow. The model permits the input of waste water discharges, tributary ¯ows, incremental ¯ows and withdrawals. A complete discussion of the model's capabilities is available in the QUAL2E model documentation (Brown and Barnwell, 1987). The conceptual representation of the river involves an idealization of the prototype as a string of completely mixed reactors or computational elements linked sequentially to one another. These elements are de®ned through mathematical formulations and connected either physically or functionally, as integral parts of the whole. A sequential group of these elements can be de®ned as reaches in which the computational elements have the same hydraulic characteristics, stream slope, channel cross section, roughness and biological and chemical rate constants. The steady state equations in QUAL2E allow the input of the hydraulic characteristics of the river reaches as empirical equations: u ˆ aQb

…1†

D ˆ cQd

…2†

where, u = stream velocity (m/s); Q = stream ¯ow (m3/s); D = stream depth (m); and a, b, c and d are empirical constants. Alternatively, given the relationship between depth and ¯ow, Manning's equation may be used to de®ne the stream velocity. The DO balance in a stream is a function of the internal sources and sinks and is represented by the following equation: u dO ˆK2 …O* ÿ O† ‡ …a3 m ÿ a4 r†A ÿ Kd L ÿ K4 =D dx ÿ a5 b1 N1 ÿ a6 b2 N2

…3†

where, u = stream velocity (m/day); x = stream distance (m); O = concentration of dissolved oxygen (mg/l); O* = saturation concentration of dissolved oxygen at the local temperature and pressure (mg/l); a3=rate of oxygen production per unit of algal photosynthesis (mg-O/mg-A); a4=rate of oxygen uptake per unit of algae respired (mg-/mg-A); a5=rate of oxygen uptake per unit of ammonia nitrogen oxidation (mg-O/mg-N); a6=rate of oxygen uptake per unit nitrite nitrogen oxidation (mgO/mg-N); m = algal growth rate (dayÿ1); r = algal respiration rate (dayÿ1); A = algal biomass concentration (mg-A/l); L = ultimate concentration of carbonaceous BOD (mg/l); Kd=carbonaceous BOD deoxygenation rate based on BOD stream pro®le (dayÿ1); K2=reaeration rate (dayÿ1); K4=sediment oxygen demand (g-O/m2-day); D = stream depth (m); b1=ammonia oxidation rate coecient (dayÿ1); b2=nitrite oxidation rate coecient (dayÿ1); N1=ammonia nitrogen concentration (mg-N/l); and N2=nitrite nitrogen concentration (mg-N/l).

Dissolved oxygen modeling

The growth and decay kinetics of algae are complex and involve many parameters in the mathematical formulations. Chlorophyll a, a component of algae, is used as an indicator to simulate algal kinetics. The algal biomass is then estimated based on the ratio of chlorophyll a to algal biomass. The change of algal biomass is represented in the model by: dA s1 ˆ mA ÿ rA ÿ A D dt

…4†

where s1=local settling rate (m/day). The algal settling rates can be input by reach. The algal growth rate, m, is a function of light, nitrogen, and phosphorus.

2403

Flow pro®le development and validation Flow pro®les were developed for the three 1991 surveys under dry weather conditions using ¯ow data from the three permanent USGS gaging stations and the point sources. The average daily ¯ows for the three permanent USGS gaging stations were calculated from the recorded hourly ¯ow data (Gadoury et al., 1991 and Socolow et al., 1992). Additional ¯ows were also measured at four temporary locations along the main stem of the Blackstone River. Point source discharges were received from the respective agencies. A summary of all measured ¯ows is presented in Table 2. The ®rst step in developing the ¯ow pro®le was the calculation of the incremental ¯ow factor, q: qˆ

RESULTS AND DISCUSSION

Blackstone River model representation The Blackstone River was divided into 25 reaches. Computational elements were 0.32 km (0.20 miles) in length. The division of reaches represents contiguous sections of the river with similar hydraulic or chemical characteristics. Reach divisions are consistent with previous modeling e€orts (Wright, 1987). Nineteen dams were de®ned in the model. The system hydraulic characteristics, equations 1) and (2, were de®ned in earlier e€orts for the MA section of the river (Reaches 1±14) by Massachusetts Department of Environmental Quality Engineering Ð MADEQE (1985), and for the RI section of the river (Reaches 18±25) by Wright (1987) and Ecology and Environment (1988). A complete list of ¯ow coecients for the reaches is provided in Table 1. Table 1. Reach hydraulic characteristics for the Blackstone River QUAL2E model Reach #

a

b

c

d

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25

0.073 0.250 0.082 0.072 0.161 0.011 0.063 0.058 0.009 0.010 0.074 0.625 0.771 0.537 0.012 0.012 0.012 0.012 0.071 0.010 0.012 0.007 0.008 0.008 0.009

0.494 0.320 0.308 0.334 0.356 0.827 0.447 0.574 0.736 0.713 0.335 0.155 0.108 0.127 0.581 0.581 0.581 0.581 0.523 0.581 0.581 0.710 0.870 0.701 0.746

0.530 0.822 0.790 4.000 0.280 3.370 0.030 0.733 0.537 0.411 0.509 0.064 0.179 0.448 1.452 1.452 0.854 1.554 0.351 1.037 0.644 0.642 0.638 0.638 0.638

0.221 0.109 0.280 0.000 0.448 0.080 0.963 0.295 0.329 0.413 0.331 0.761 0.528 0.379 0.400 0.400 0.400 0.400 0.400 0.400 0.400 0.400 0.400 0.400 0.400

QW ÿ …QQ ‡ QB ‡ QWWTP † DAW ÿ …DAQ ‡ DAB †

…5†

where QW=¯ow at the Woonsocket USGS gage, RI (BLK17) (m3/s); QQ=¯ow at the Quinsigamond River USGS gage in North Grafton, MA (upstream BLK05) (m3/s); QB=¯ow at the Branch River USGS gage in Forestdale, RI (BLK14) (m3/s); QWWTP=discharges from all waste water discharges above the Woonsocket wastewater treatment plant (WWTP) (m3/s); DAW=drainage area at the USGS Woonsocket gage (1065 km2, 4l6 mi2); DAQ=drainage area at the USGS Quinsigamond gage (65.5 km2, 25.6 mi2); and DAB=drainage area at the USGS Branch River gage (233 km2, 91.2 mi2). The incremental ¯ow, q, was estimated as 0.014 (0.192), 0.015 (0.201), and 0.092 (1.271) m3/ km2-s (cfs/mi2) for the July, August and October 1991 surveys, respectively. The predicted tributary and reach ¯ows were determined by multiplying drainage area and q. These predicted ¯ows were veri®ed with the USGS independent ¯ow data collected at the temporary gage stations at BLK01, BLK07, BLK12 and BLK20, and at the permanent gage station at BLK17 (Table 2). The relative error (R.E.) of the observed and predicted ¯ows was computed as: R:E: ˆ

jX ÿ Cj  100% X

…6†

where X = observed ¯ow (m3/s) and C = predicted ¯ow (m3/s). The median relative errors for the July, August and October, 1991 surveys were 2.3, 5.9 and 9.6%, respectively. The ¯ow pro®les were also evaluated through the simulation of chloride. Chloride is a conservative constituent which experiences no losses due to biological, physical, or chemical actions. Dilution is the only mechanism that a€ects its concentration and therefore, modeling success supports the ¯ow pro®le. A graphical plot of the observed versus predicted concentration of chloride for the three 1991 surveys is shown in Fig. 2. The median relative

2404

Rajat R. Chaudhury et al. Table 2. Summary of measured ¯ows for the 1991 dry weather surveys Station

Flowd (m3/s)

Location July 10±11 b

U.S. Steel (BLK01) Northbridge (BLK07)b Millville (BLK12)b Woonsocket (BLK17)a Lonsdale (BLK20)b Quinsigamond Rivera Branch Rivera UBWPAD Millbury Grafton Northbridge Uxbridge Woonsocket

Blackstone River

Tributaries WWTPc

0.382 2.19 2.79 3.88 5.35 0.207 0.736 1.09 0.017 0.046 0.050 0.110 0.235

August 14±15 0.396 2.39 3.34 4.30 5.66 0.242 0.863 1.26 0.024 0.046 0.035 0.110 0.325

October 2±3 1.96 6.68 13.7 18.0 21.5 1.71 3.45 1.83 0.037 0.045 0.048 0.110 0.379

a

USGS permanent gaging stations. USGS temporary gaging stations. Wastewater treatment plant. d Flow average of both days. b c

errors for July, August and October 1991 surveys were 6.1, 19 and 7.0%, respectively. Based on the observed and predicted ¯ows for the USGS gage stations and chloride simulations, the model predictions were excellent and the ¯ow pro®les are considered calibrated and veri®ed. For the validation survey in August, chloride simulations were not as successful due to unexpected high variations in the UBWPAD WWTP discharge (Table 3) over the ®ve days leading to the survey. The model is particularly sensitive to UBWPAD discharge concentrations, since UBWPAD discharge

accounts for approximately 75% of the ¯ow in the river at its point of discharge for the July and August surveys. Dissolved oxygen model calibration Incremental in¯ow concentrations. Since the Branch and Mumford Rivers had similar chloride concentrations and the lowest among the tributaries, their mean concentrations from the calibration surveys of July and October surveys were averaged. The resulting average value of 17.5 mg/l was adopted for all reach in¯ows. Based on the suc-

Fig. 2. Observed and predicted chloride concentrations for 1991 surveys.

Dissolved oxygen modeling

2405

Table 3. Pollutant inputs of point sources to the Blackstone River Parameter (mg/l)

Chloride CBOD5 Ammonia Nitrate Phosphorus

Point source

UBWPAD WWWTP UBWPAD WWWTP UBWPAD WWWTP UBWPAD WWWTP UBWPAD WWWTP

July 10±11

August 14±15

October 2±3

Min.

Max.

Min.

Max.

Min.

Max.

88.0 100 2.00 3.70 0.100 27.7 5.70 0.900 1.60 3.00

96.0 154 5.00 7.60 1.10 28.6 7.10 1.00 3.00 3.90

62.0 175 3.80 6.15 0.100 9.00 6.00 2.00 1.90 4.90

132 290 5.70 14.4 0.600 16.8 31.6 58.5 3.00 3.35

73.0 191 1.73 7.65 0.100 7.70 9.80 1.4 2.40 3.70

141.0 440 4.70 20.0 0.800 15.8 13.4 4.70 3.40 4.20

UBWPAD provides seasonal advanced treatment (nitri®cation). Woonsocket WWTP provides secondary treatment (activated sludge).

cess of the chloride simulations, the average concentrations of the Branch and the Mumford Rivers for the July and October surveys were also used to represent background conditions for dissolved ammonia, nitrate, and phosphorus (0.05, 0.18 and 0.06 mg/l, respectively). DO concentrations for incremental in¯ows were input at 75% of saturation DO based on previous DO modeling e€orts (Wright et al., 1989). CBOD was set to zero, since the Branch and Mumford Rivers concentrations for July and October were below the detection limit of 1.0 mg/l. Atmospheric reaeration rate (k2). Based on stream velocity and depth in the river, the O'Connor and Dobbins (1958) equation was selected for development of the reaeration coecient. All dams were modeled as sharp crested weirs. The reaeration due to the nineteen dams had a signi®cant impact on DO. A net positive e€ect occurs when the impounding water has a DO less than 90% saturation and a net negative e€ect occurs when DO is supersaturated due to algal photosynthesis. In either case, the return DO concentrations in the river was about 90% of the DO saturation value. CBOD simulations. The slope of the semi-log plot of 5 day CBOD ¯ux (kg/day) and travel time is the rate of BOD removal (Kd). For the majority of reaches, Kd could not be calculated. This was due to the ®ve day CBOD concentrations in the river being close to the detection limit of 1.0 mg/l, typically ranging between 1.0±2.0 mg/l. Thus, a Kd of 0.10 dayÿ1 base e at 208C with a temperature correction factor of 1.047 was used in all reaches of the Blackstone River to represent a minimum decay rate in a stream system (Wright and McDonnell, 1979). The rate of loss of CBOD due to settling was considered to be insigni®cant. Average BOD5 concentrations observed were used for all tributaries, WWTPs (Table 3) and the headwaters for July and October. Minor adjustments were made to the UBWPAD input within the 95% con®dence limit of the data to have the model predictions within the range of observations. BOD loadings from the point sources are minor and most of the observed BOD downstream of km 56.0 (mile

35.0) for the July and August surveys are thought to be due to detrital phytoplankton carbon from algae. Nitrogeneous BOD. Observed organic nitrogen concentrations in the Blackstone River were low and were generally below the detection limit of 1 mg/l. Due to the relative insigni®cance of organicN concentrations on other nitrogen species and dissolved oxygen concentrations, average literature values of 0.2 dayÿ1, 0.05 dayÿ1, and 0.100 mg-N/ mg-A were used to represent the coecients for hydrolysis of organic nitrogen to ammonia nitrogen, organic nitrogen settling and fraction of algal biomass that is nitrogen, respectively. The rate coecients b1 and b2 are used to represent the conversion of ammonia to nitrite and nitrite to nitrate, respectively. Similar to CBOD deoxygenation rates, the ammonia ¯ux was plotted against time of travel on semi-log plots for the summer survey in July. The slope of this plot is established as b1. The term b2 was estimated as twice the value of b1 as suggested in QUAL2E. The range of b1 values suggested in QUAL2E is 0.10 to 1.0 dayÿ1 base e at 208C. River reaches, where b1 could not be estimated, were set to the lower limit of 0.10 dayÿ1 base e at 208C. River reaches, where b1 values were above the upper limit, were input as 1.0 dayÿ1 base e at 208C. For other river reaches, b1 values varied from 0.30 to 0.88 dayÿ1 base e at 208C. Temperature correction coecients for b1 and b2 used are 1.083 and 1.047, respectively. Ammonia simulations for the July survey are shown in Fig. 3 and indicate excellent model predictions downstream of Woonsocket where dissolved ammonia is most signi®cant. For both the August and October surveys, the same coecients b1 and b2 were used. The model predictions also provided an excellent ®t to the observations. Sediment oxygen demand (SOD). SOD rates were measured at 10 stations along the Blackstone River in MA and RI. The observed values varied from 1.61 to 5.92 g-O/m2-day (0.15 to 0.55 g-O/ft2-day). Usually, the highest values were found at impoundments and are typical of ranges found by Thomann

2406

Rajat R. Chaudhury et al.

Fig. 3. Ammonia simulations for July 10±11, 1991.

and Mueller (1987) for sections located in the vicinity of sewage outfalls. SOD values have a signi®cant impact upstream of the dams for the July survey. However, this impact is limited since the reaeration over the dams causes the DO concentration to recover. For the October Survey, the SOD impacts are not as signi®cant, due to the higher dilution, lower temperatures and shorter detention times (higher velocities). For the validation survey of August, DO impacts from SOD are similar to July. Algal productivity. The simulations of chlorophyll a and algal growth constituted the ®nal stage of the calibration process to re¯ect the uptake of nitrates and phosphates in the water column. Since many of the parameters required in the simulation of algal growth are not measured, a range of values suggested in QUAL2E were tested systematically. The ®nal values of parameters used for the calibration and validation are listed in Table 4. The

remaining parameters used for algal simulations represent an averaged value of the range suggested in the QUAL2E manual (Brown and Barnwell, 1987). The chlorophyll a simulations for July are shown in Fig. 4. The nitrate and phosphate simulations are depicted in Fig. 5 and 6, respectively. All simulations show a good correlation between observed and predicted data. Dissolved Phosphate concentrations decrease after mile 25 where chlorophyll a concentrations begin to increase. Discrepancies exist between the July and August surveys with respect to the uptake rate of dissolved phosphorus and the chlorophyll a concentrations. The August survey indicated phosphorus limiting conditions from km 40.0 (mile 25.0) to km 25.6 (mile 16.0) with 8 of the 12 samples below the detection limit of 0.02 mg/l-P. The major sources of phosphorus are from the UBWPAD and Woonsocket facilities (Table 3).

Table 4. Final values of parameters used in algal simulations Parameter Ratio of chlorophyll a to algal biomass Nitrogen content of algal biomass Phosphorus content of algal biomass Maximum algal growth rate Algal respiration rate Organic phosphorus decay rate Organic phosphorus settling rate Benthos source for dissolved phosphorus a

Bowie et al. (1985). Brown and Barnwell (1987).

b

Unit

Range Ia

Range IIb

mg-chl a/mg-A mg-N/mg-A mg-P/mg-A dayÿ1 dayÿ1 dayÿ1 dayÿ1 mg/m2-day

2.5±100 0.06±0.16 0.002±0.05 0.058±9.2 0.02±0.92 0.003±0.70 0.001±0.8 0.0004±1.7

10±100 0.07±0.09 0.01±0.02 1.0±3.0 0.05±0.50 0.01±0.70 0.001±0.01 variable

Model input 3.0 0.10 0.05 2.50 0.20 0.35 0.05 0.50

Dissolved oxygen modeling

Fig. 4. Chlorophyll a simulations for July 10±11, 1991.

Fig. 5. Dissolved nitrate simulations for July 10±11, 1991.

2407

2408

Rajat R. Chaudhury et al.

Fig. 6. Dissolved phosphorous simulations for July 10±11, 1991.

Fig. 7. Final dissolved oxygen pro®le for July 10±11, 1991.

Dissolved oxygen modeling

Nitrate simulations, however, do not illustrate these discrepancies and track the observed concentration pro®les. Final dissolved oxygen pro®les. With the model calibrated for algal productivity, the ®nal DO pro®les can be plotted for the three surveys. Figure 7 represents the ®nal dissolved oxygen pro®le for the July survey. This survey showed the highest algae productivity among the three surveys. The plot shows the inclusion of the sources and sinks to the model and represents the average DO values for a 24 h period. Statistical tests were done to evaluate the model performance in simulating DO for the Blackstone River. The median relative error of the model predictions for the July, October and August 1991 surveys were 6.7%, 7.8% and 11.8% and indicate the good credibility of the model. In a review of 15 DO water quality models by Thomann (1982), it was found out that the median relative error was approximately 10%. Model validation The QUAL2E model for the Blackstone River has been calibrated with two data sets collected in July and October 1991 and validated with one data set from the August Survey. An additional validation is provided with a previous water quality data set collected by MADEQE in 1983. This data

2409

set was originally used to calibrate another model (Stream7). Monitoring for DO, BOD, ammonia and dissolved phosphorus for the Stream7 model calibration occurred on June 9±13, 1980, August 4±8, 1980 and October 15±16, 1980. Sampling included stations BLK01 to BLK12 and UBWPAD. DO was monitored for two days at six hour intervals, while dissolved ammonia and phosphorus samples were collected twice for each survey. The application of the calibrated QUAL2E required ¯ow pro®les to be developed using the average ¯ows at the three USGS stations. All boundary conditions to the QUAL2E model are given as the mean of the observed values (MADEQE, 1983). QUAL2E was run dynamically to include the impact of algal productivity, and the results are given as a daily average of four six-hour interval simulations (similar to observed values). Observed and simulated DO pro®les are shown in Fig. 8 and represent the mean DO values for the day for the June 9±12, 1980 survey. QUAL2E successfully simulated the 1983 DO pro®les. It should be noted that Stream7 simulations had been performed without productivity. Stream7 was used in a 1983 waste load allocation which resulted in the expansion of the UBWPAD to include seasonal nitri®cation. To further test the performance of QUAL2E, the 1980 ammonia pro®le, depicted in Fig. 9, is also included. QUAL2E model simulations showed excellent predictions of

Fig. 8. Dissolved oxygen pro®le for validation survey (MADEQE, 1983), June 9±13, 1980.

2410

Rajat R. Chaudhury et al.

Fig. 9. Ammonia pro®le for validation survey (MADEQE, 1983), October 15±16, 1980.

instream observations of ammonia for conditions prior to UBWPAD's upgrade to nitri®cation. While a comprehensive post audit of the Stream7 model is not performed here, the ability of the QUAL2E model to predict conditions before and after UBWPAD's improvement suggests that the 1983 modeling e€ort was adequate and consequently, the enhanced treatment has had a signi®cant impact on DO concentrations in the river. Model application for low ¯ows A model application with critical low ¯ow (7Q10) is included in this section. The 7Q10 incremental in¯ows were generated by using equation 5. These in¯ows with the WWTP permit ¯ows were used to develop the 7Q10 pro®le for the river. This ensured minimal ¯ows from the tributaries and reaches for dilution. The condition listed below evaluates the response of the river to changes in discharge quality from UBWPAD. The baseline condition simulated in QUAL2E has all WWTP inputs to the model set at the maximum daily discharge permit concentrations. For the Woonsocket WWTP, since a permit for ammonia does not exist, the ammonia input of 28.2 mg/l (Table 3) represents the average of the July survey, the highest average concentration monitored during the three 1991 surveys. Tributary concentrations represent the average of the July and August 1991 surveys. Simulations were performed with and without productivity in order to isolate

the key sinks causing oxygen depletion, while also addressing the issue of the impact of limiting productivity through discharge regulation. Violations of the 5.0 mg/l DO criteria in MA occur at the impoundments upstream of Singing Dam (River km 24.9 Ð mile 39.8), Fisherville Dam (River km 22.8 Ð mile 36.5), Riverdale Dam (River km 19.9 Ð mile 31.9) and Rice City Pond Dam (River km 17.4 Ð mile 27.8) with and without productivity (Fig. 10). The DO consumption occurs due to SOD. DO violations occur in RI due primarily to nitri®cation downstream of the Woonsocket WWTP. The lowest DO concentrations are predicted upstream of the Central Falls Dam (River km 1.25 Ð mile 2.00). DO violations are greater without the presence of productivity, showing the dominance of net bene®t of higher photosynthesis over respiration.

CONCLUSIONS AND SUMMARY

The following conclusions were determined from this analysis: . The median relative error of the dissolved oxygen model predictions for the July, October and August 1991 surveys were 6.7%, 7.8% and 11.8%; . The impoundments along the river serve to reduce velocities and increase the time of travel in the river reaches directly behind the dams. These conditions compound the problems due to high

Dissolved oxygen modeling

2411

Fig. 10. Dissolved oxygen pro®le for 7Q10 ¯ow condition.

levels of phosphorus by providing the appropriate hydraulic conditions for the growth of algae; . The river reaches with the highest nitri®cation rates are directly below the Woonsocket WWTP. Instream nitri®cation governs the oxygen pro®les in these reaches and causes a dissolved oxygen sag below Woonsocket's discharge that often extends to the mouth of the river in Pawtucket, RI; . The nineteen impoundments along the river are sediment traps for dry weather conditions. The sediments behind these impoundments are a major sink of oxygen often dominating the oxygen pro®le. This is especially true in the upstream reaches where productivity and instream nitri®cation are relatively small compared with the lower reaches; . Model application to the 7Q10 ¯ow showed the potential of major violations of dissolved oxygen criteria of 5.0 mg/l downstream of the Woonsocket WWTP. The major cause of these violations is the instream nitri®cation due to the high discharges of ammonia from this facility; . Model application to the 7Q10 ¯ow showed minor violations in MA upstream of four impoundments. The major causes of these violations are high sediment oxygen demands; . Based on a comparison of data from the early 1980s and this study and the model application, it was clear that the advanced wastewater treatment implemented in the mid 1980s at UBWPAD has made a signi®cant improvement to the dissolved oxygen concentration in the river;

. A post audit of the waste load allocation conducted in 1983 for the UBWPAD was successful. The model's application and the modeler's interpretation of the results were appropriate. AcknowledgementsÐThe authors thank the United States Environmental Protection Agency (USEPA), the Massachusetts Department of Environmental Protection (MADEP) and the Rhode Island Department of Environmental Management (RIDEM) for providing ®nancial support for this research. REFERENCES

American Public Health Association, American Water Works Association, and Water Environment Federation (1989) Standard Methods, Washington, DC. Bowie G. L., et al. (1985) Rates, Constants, and Kinetics Formulations in Surface Water Quality Modeling, 2nd edn. EPA/600/3-85/040, Athens, GA. Brown L. C. and Barnwell Jr., T. O. (1987) The Enhanced Stream Water Quality Models QUAL2E and QUAL2EUNCAS: Documentation and User Manual. Environmental Research Laboratory, Oce of Research and Development. United States Environmental Protection Agency, Athens, GA. Ecology and Environment Inc. (1988) E€ects of Ocean State Power Water Withdrawal on Dissolved Oxygen in the Blackstone River, Bu€alo, NY. Gadoury R. A., Socolow R. S., and Ramsbey L. R. (1991). United State Geological Survey (USGS) Water Resources Data Report for Massachusetts and Rhode Island, USGS, Marlborough, MA. Hartman E. (1992) Blackstone River Initiative Dry Weather Results, Massachusetts Department of Environmental Protection, Grafton, MA.

2412

Rajat R. Chaudhury et al.

Massachusetts Department of Environmental Quality Engineering (1983) Upper Blackstone Water Pollution Abatement District, Waste Load Allocation, Division of Water Pollution Control, Westborough, MA. Massachusetts Department of Environmental Quality Engineering (1985) Blackstone River Basin, Water Quality Survey Data and Wastewater Discharge Survey Data, Division of Water Pollution Control, Westborough, MA. O'Connor D. J. and Dobbins W. E. (1958) Mechanics of reaeration in natural streams. Trans. ASCE 123, 641± 648. Socolow R. R., Shepard T. S., Girouard G. G., and Gadoury R. A. (1992) United State Geological Survey (USGS) Water Resources Data Report for Massachusetts and Rhode Island, USGS, Marlborough, MA. Thomann V. R. (1982) Veri®cation of water quality models. Journal of the Environmental Engineering Division, ASCE 108, 923±940.

Thomann V. R. and Mueller J. A. (1987) Principles of Surface Water Quality Modeling and Control. Harper and Row Publishers, New York, NY. Wright R. M. and McDonnell A. J. (1979) In-stream deoxygenation rate prediction. Journal of the Environmental Engineering Division, ASCE 105, 323± 335. Wright R. M. (1987) Development of a One-Dimensional Water Quality Model for the Blackstone River, Part 2: Mathematical Modeling, Narragansett Bay Project, University of Rhode Island, Kingston, RI. Wright R. M., et al. (1989) A Water Quality Impact Analysis of a Hydropower Facility, Proceedings of Waterpower `89 Proceedings International Conference on Hydropower, U.S. Army Corps of Engineering/ ASCE, Niagara Falls, NY, August 23±25. Wright R. M., et al. (1991) Problem Assessment and Source Identi®cation and Ranking of Wet Weather Discharges Entering the Providence and Seekonk Rivers, Narragansett Bay Project, Providence, RI and USEPA, Boston, MA.