Light-scattering Features of Turbidity-causing Particles in Interconnected Reservoir

Basins and a Connecting Stream

UpstateFreshwaterInst.

Feng Peng and Steven W. EfflerUpstate Freshwater Institute, Syracuse, New York

Donald C. PiersonNYC Department of Environmental Protection

David G. SmithNational Institute of Water and Atmosphere, New Zealand

Study System turbidity as a water quality issue

Character of Light-scattering (Turbidity-causing) Particles within the Catskill System

Related questions

What are the light-scattering characters of particles (size distributions, composition, shape) in the Catskill system?

Are there differences in the light scattering characteristics of particles in different parts of the Catskill System?

Does the potential difference cause disproportionate contributions of the source particles to turbidity (Tn) within the system?

Turbidity (Tn): A Measure of Light Scattering

(light scattering coefficient, b; m1)

Tn measured at acceptance angle centered at 90 (“side-scattering”)

Tn b

0º

90º

Light scattering coefficient (b) depends on four features of particle population

1. particle number concentration (N)2. particle size distribution (PSD)3. particle composition (i.e., refractive index)4. particle shape

Particle characterizations• bulk measurements of mass (TSS) and mass

fractions; disconnect with light scattering• PSDs counters; size limitations, no chemical

composition• SAX§ individual particle analysis; N, PSD,

composition, and shape

Particles and Light Scattering : Dependencies and Analytical Support

§ scanning electron microscopy interfaced with automated image and X-ray analyses

Individual Particle Analysis (IPA)

by Scanning electron microscopy

interfaced

with Automated image and X-ray analyses

(SAX)

Detailed compositional and morphological analyses

SAX Characterizations§

• Chemical (elemental X-rays) 5 inorganic particle types,

including clay minerals, quartz

• Morphological rotating chord algorithm

PAi sum of all triangular areas

di area equivalent diameter

shape “nonsphericity”,

ASP (aspect ratio) = Dmax/Dperp

• >1,000 particles analyzed in each sample

§ Peng and Effler (2007) Limnol. Oceangr. 52: 204216. Peng et al. (2009) Water Res. 43: 22802292.

0.0 0.5 1.0 1.5 2.0 2.5 3.00

500

1000

1500Si

X-r

ay C

ounts

keV

Al

V sample volume N number of particles per unit volume of waterQb,i light scattering efficiency of particle i; Mie theory

mi (complex) relative refractive index of particle i

(ni in); depending on composition

wavelengthdi size of particle i

PAi projected area of particle i

Calculation of b from SAX Measurements

according to light scattering theory

b,1

1( ) ( , , ) PA

N

i i i ii

b Q m dV

light

System Configuration,

and Sampling (2005) for

SAX Characterizations

Sites (n = 9)

Schoharie site 3 and withdrawal

Esopus AP (above portal), E16i

Ashokan sites 3 and 1 (W. basin),

site 4 (E. basin)

Kensico sites 4.2 and 4.1

Runoff Conditions (Q)

low Q and Tn; high Q and Tn

Tunnel Operations

on/off

SchoharieRes.

AshokanRes.

KensicoRes.

EsopusCreek

ShandakenTunnel

CatskillAqueduct

Dependency of b on Size(calculated from SAX results)

Particle Size Distribution(SAX observation) Esopus Creek, E16i 13 Apr 2005Tn 66.7 NTU

50%

d50 = 2.70 m

d25

d75

Scenarios of Interest in the Catskill System related to the relative contribution of Schoharie Reservoir (diversions, Shandaken Tunnel)

Evaluating the potential for Tn from Schoharie Reservoir (Tn/SCH)

making a disproportionately large contribution to Tn leaving the

east basin of Ashokan (Tn/ASH) to Kensico

2. shape: ASPSCH >> ASPEsop ??

i.e., greater deviation from sphericity

3. size: d50/SCH << d50/Esop ??

Systematically smaller particles would settle more slowly (i.e., persistence)

1. composition (i.e., refractive index):

EsopSCH ?? n n ??

Particle Composition for Sites throughoutthe Catskill Systems

SitesNo. ofSampl

es

Tn

rangeb(660) Range % of b (mean ± std. dev.)

(NTU) (m1) Clay Quartz Si-rich Fe/Mn Misc.

SCH R. 2002

53 4.281 2.446.482.5 ±

3.18.3 ± 2.7

3.8 ± 1.2

1.7 ± 0.8

3.6 ± 1.6

2005

92.2 440

0.8203.786.1 ±

7.58.1 ± 5.8

2.8 ± 1.3

1.0 ± 0.5

1.9 ± 0.6

AP 2005 6 1.576 0.744.476.6 ±

2.912.5 ±

2.84.9 ± 2.2

1.2 ± 0.4

4.6 ± 3.4

E16i 2005 4 2.767 1.044.977.6 ±

4.613.4 ±

4.45.0 ± 1.2

1.0 ± 0.7

3.0 ± 0.9

ASH W. 2005

6 1.6464 0.7336.276.8 ±

2.214.8 ±

2.65.0 ± 1.5

1.1 ± 0.4

2.3 ± 0.6

ASH. E. 2005

22.631.

81.015.9 78.0 ± -- 13.4 ± -- 4.9 ± -- 1.0 ± -- 2.7 ± --

Kensico 2005

2 1.622 0.814.3 79.5 ± -- 11.8 ± -- 4.9 ± -- 1.5 ± -- 2.3 ± --EsopSCH )(n nAnswer to question 1: compositionally uniform

Particle Shapes (ASP Values) for Sites throughout the Catskill Systems

SitesSample

sASP

n mean ± std. dev.

Schoharie R. 2002

53 1.75 ± 0.09*

2005

9 2.16 ± 0.2

Esopus AP 2005 6 1.90 ± 0.08Esopus E16i 2005

4 1.90 ± 0.03

Ashokan R. W. 2005

6 2.03 ± 0.2

Ashokan R. E. 2005

2 1.98 ± --

Kensico R. 2005 2 2.35 ± --* ‘Clay’-type particles only (Peng and Effler, 2007. Limnol. Oceanogr.)

Answer to question 2: similar morphology (ASPSCH ASPEsop)

Uniformity of Particle Size Distributions in the Catskill System in the Context of Light Scattering (i.e., Tn)

Apr 2005, wet conditions Relatively minor variations in sizes regulating b and therefore, Tn

Quartile Sizes (μm)

d25 d50 d75

SCH R. 1.71 2.54 4.17

Intake 1.67 2.43 3.88

AP 1.84 2.74 4.69

E16i 1.78 2.66 4.52

Comparison of Particle Size Contributions to b (Tn) Esopus Creek example

Very similar particle size dependencies over stream length

0.1 1 10 40

0

5

10

0

20

40

60

80

100

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Size (m)

Tn

A AP, 76.1E E16i, 66.7

b(66

0) P

ct.

13 Apr 2005(tunnel off)

Cum

. b(6

60)

Pct

.

0.1 1 10 400

5

10

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Size (m)

b(66

0)

Pct

.

A APE E16i

19 Jul 2005(tunnel on)

1 AP2 E16i

13 Apr 2005(tunnel off)

Comparison of Particle Size Contributions to b (Tn) Esopus Creek example, tunnel on/off

Tn

AP, 18.8E16i, 23.0

0.1 1 10 400

5

10

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1

1

111

1

1

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1

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1

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2

2

2

22

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2

2

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2

Size (m)

b(66

0)

Pct

.

A APE E16i

19 Jul 2005(tunnel on)

1 AP2 E16i

13 Apr 2005(tunnel off)

AP, 76.1E16i, 66.7

Quartile Sizes of Scattering for Sites throughout the Catskill Systems

(Wet Conditions, Apr 2005)

SchoharieRes.

AshokanRes.

KensicoRes.

EsopusCreek

ShandakenTunnel

CatskillAqueduct

0

100

200

300

400

500

0

1

2

3

d 5

0 of b(

660)

(m

)

Wet conditions

AP SCH SCH E16i AsW AsW AsE KenSta3 Intake Sta1 Sta3 Sta4 Sta4.2

Turb. (N

TU

)

Tn

AP SCH SCH E16i AsW AsW AsE KenSta3 Intake Sta1 Sta3 Sta4 Sta4.2

0

2

4

quar

tile

size

s of

b(6

60)

(m

) d

25 d

50 d

75

Answer to question 3:

The analyses of PSDs and the size dependency patterns of b indicate that

– particles from Schoharie Intake were not noticeably smaller than those from the Esopus watershed.

SAX-based Estimate as Strong Predictor of Turbiditytechnique credibility

Significance:

• SAX provides representative specifications of Tn-causing attributes of particles in the Catskill System

• SAX can be used to address the issue of potential heterogeneity in light scattering and settling within this system Good closure

0 100 200 3000

100

200

300

400

500

y = 1.63*x + 2.38

R2 = 0.94n = 29

Tn (

NT

U)

b(660) (m)

Summary• Highly uniform light-scattering (thus turbidity-causing)

properties of the suspended particles throughout the Catskill system over a wide range of turbidity – composition (clay minerals dominating)– size distribution– shape

• Similar potencies of particle populations in upstream vs. downstream turbidity sources

• Findings support– direct incorporation of Tn measurements into loading

calculations to evaluate source impacts– parameterization of mechanistic turbidity models; e.g.,

representations of particles in the turbidity models for Schoharie, Ashokan, and Kensico

• Published manuscriptPeng, F., S.W. Effler, D. C. Pierson, and D. G. Smith. 2009. Light-scattering features of turbidity-causing particles in interconnected reservoir basins and a connecting stream. Water Research 43(8): 2280–2292.