Compendium of In Situ Pore-Liquid Samplers for Vadose Zone

Chapter 19

Compendium of In Situ Pore-Liquid Samplers for Vadose Zone 1

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David W. Dorrance , L. G. Wilson , L. G.

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Everett , and S. J. Cullen

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ENSR, 3000 Richmond Avenue, Houston, TX 77098 Department of Hydrology and Water Resources, University of Arizona, Tucson, AZ 85721 Metcalf and Eddy, 816 State Street, Santa Barbara, CA 93102-0551 Institute for Crustal Studies, University of California, Santa Barbara, CA 93106

Downloaded by UNIV OF ARIZONA on August 6, 2012 | http://pubs.acs.org Publication Date: June 20, 1991 | doi: 10.1021/bk-1991-0465.ch019

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In recent years, there has been increasing emphasis on monitoring contaminant transport i n the vadose zone. Vadose zone monitoring r e l i e s on a variety of i n s i t u samplers to c o l l e c t pore-liquids under saturated and/or unsaturated conditions. This compendium describes these samplers together with t h e i r advantages and disadvantages. The vadose zone i s t h e h y d r o g e o l o g i c a l r e g i o n extending from the l a n d s u r f a c e t o the p r i n c i p l e water t a b l e . Other commonly used terms f o r t h i s r e g i o n are the "unsaturated zone" and the "zone o f a e r a t i o n " . These a l t e r n a t i v e terms do not take i n t o account the e x i s t e n c e o f s a t u r a t e d flow above t h e p r i n c i p l e water t a b l e . Saturated o r nears a t u r a t e d flow occurs p r i m a r i l y under t h e i n f l u e n c e o f gravity ( r e f e r r e d t o as f r e e d r a i n a g e ) . Under some c o n d i t i o n s , p o r e - l i q u i d s may c o l l e c t on p e r c h i n g l a y e r s , and l o c a l l y s a t u r a t e d c o n d i t i o n s (perched ground water) may develop. Unsaturated p o r e - l i q u i d flow through the vadose zone under unsaturated c o n d i t i o n s i s c o n t r o l l e d p r i m a r i l y by negative p o r e - l i q u i d pressure g r a d i e n t s (negative porel i q u i d pressures are r e f e r r e d t o as p o r e - l i q u i d t e n s i o n s o r matric p o t e n t i a l ) . Chemical s p e c i e s r e l e a s e d a t or near the land s u r f a c e w i l l be t r a n s p o r t e d t o some degree through the vadose zone. E x t r a c t i o n and chemical analyses of vadose-zone l i q u i d s i s regarded as an e a r l y warning approach t o p o t e n t i a l groundwater p o l l u t i o n from such r e l e a s e s . T h i s i n f o r m a t i o n can be used t o m i t i g a t e p o t e n t i a l problems p r i o r t o ground water degradation (1-2). Vadose-zone l i q u i d s can be e x t r a c t e d from cores i n t h e l a b o r a t o r y or, a l t e r n a t i v e l y , porel i q u i d s may be sampled d i r e c t l y from "undisturbed" s o i l s by i n s t a l l i n g i n s i t u p o r e - l i q u i d samplers. 0097-6156/91/0465-0300$09.00A) © 1991 American Chemical Society

In Groundwater Residue Sampling Design; Nash, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1991.


19. DORRANCE ET AL

In Situ Pore-Liquid Samplers for Vadose Zone

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The most obvious d i f f e r e n c e between these two techniques i s t h a t vadose zone sampling i s a d e s t r u c t i v e process which prevents r e p e t i t i v e sampling from t h e same l o c a t i o n . More importantly, t h e two techniques do not sample the same types o f l i q u i d (3.-4) . In s i t u samplers a r e o n l y capable o f sampling p o r e - l i q u i d s h e l d a t t e n s i o n s o f up t o about 60 kPa (5) . S o i l sampling with subsequent porel i q u i d e x t r a c t i o n p r o v i d e s l i q u i d s which may be h e l d a t t e n s i o n s o f up t o s e v e r a l bars, depending on t h e e x t r a c t i o n technique. E x t r a c t i o n under s e v e r a l bars o f p r e s s u r e may s t r i p o f f c a t i o n s p r e f e r e n t i a l l y sorbed i n e l e c t r i c a l double l a y e r s , sorbed o r g a n i c s , and even components o f the s o i l . These s p e c i e s may not be present i n t h e same c o n c e n t r a t i o n s (absolute o r r e l a t i v e ) i n samples p r o v i d e d by i n s i t u p o r e - l i q u i d samplers. T h i s chapter reviews v a r i o u s i n s i t u samplers and i n c l u d e s r e l e v a n t l i t e r a t u r e c i t a t i o n s . Some o f the d e s c r i b e d samplers a r e not commercially a v a i l a b l e a t t h i s time. However, they may have been a v a i l a b l e i n t h e past, and may be encountered a t s i t e s with e s t a b l i s h e d vadose zone monitoring programs. Some o f the samplers can be f a b r i c a t e d . There a r e numerous q u a l i f i e r s , h i n t s , and warnings which should accompany t h e d e s c r i p t i o n o f each sampler. We depend on t h e reader t o review c i t e d r e f e r e n c e s to o b t a i n complete e x p o s i t i o n s o f t h e covered samplers. The a p p l i c a t i o n s and l i m i t a t i o n s o f many o f t h e samplers presented here were d e s c r i b e d p r e v i o u s l y (S-9). In

S i t u P o r e - L i q u i d Sampler C a t e g o r i e s

In s i t u samplers e x t r a c t l i q u i d s from s a t u r a t e d and unsaturated zones. Most samplers designed t o sample from unsaturated s o i l s a l s o sample from s a t u r a t e d s o i l s . T h i s i s u s e f u l i n areas where the water t a b l e f l u c t u a t e s , r e s u l t i n g i n a l t e r n a t i n g s a t u r a t e d and unsaturated c o n d i t i o n s . In c o n t r a s t , samplers designed f o r sampling from s a t u r a t e d zones cannot be used i n unsaturated c o n d i t i o n s . T h i s i s because t h e negative p o r e - l i q u i d p r e s s u r e s i n unsaturated zones prevent l i q u i d from moving i n t o a i r - f i l l e d c a v i t i e s at atmospheric pressures (Richard's Outflow P r i n c i p l e ) . A l s o , t h e openings i n s a t u r a t e d samplers a r e t o o l a r g e t o prevent a i r from e n t e r i n g the samplers when s u c t i o n s are a p p l i e d . Using t h i s d i s t i n c t i o n , t h e types o f p o r e - l i q u i d samplers have been c a t e g o r i z e d as f o l l o w s : ο s u c t i o n samplers (unsaturated/saturated sampling) ο experimental s u c t i o n samplers (unsaturated/saturated sampling) ο experimental a b s o r p t i o n samplers (unsaturated/saturated sampling) ο f r e e drainage samplers (saturated sampling) ο perched ground water samplers (saturated sampling).


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GROUNDWATER RESIDUE SAMPLING DESIGN M

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The term p o r e - l i q u i d c o u l d be a p p l i c a b l e t o any l i q u i d r e s i d i n g i n s o i l ranging from aqueous p o r e - l i q u i d s t o o i l . However, a l l o f t h e samplers d e s c r i b e d i n t h i s paper were designed t o sample aqueous p o r e - l i q u i d s o n l y . The a b i l i t i e s o f these samplers t o c o l l e c t other porel i q u i d s may be q u i t e d i f f e r e n t than those d e s c r i b e d .


Vadose Zone M o n i t o r i n g Program Design The c h o i c e of a p p r o p r i a t e sampling d e v i c e s f o r a p a r t i c u l a r l o c a t i o n i s dependant on v a r i o u s c r i t e r i a (Table I ) . W e l l s t r u c t u r e d s o i l s have two d i s t i n c t flow r e g i o n s i n c l u d i n g macropores ( e . g . i n t e r p e d a l openings, c r a c k s , burrows, and r o o t t r a c e s ) and micropores (e.g. i n t r a p e d a l openings between s o i l g r a i n s ) . Under s a t u r a t e d c o n d i t i o n s , l i q u i d s move more rapidly through macropores than through micropores. Because o f t h i s , contaminants t r a n s p o r t e d by f r e e drainage may bypass t h e f i n e r pores. Consequently, p o r e - l i q u i d s i n macropores may have d i f f e r e n t c h e m i s t r i e s than those i n micropores (10) . T h i s i s enhanced by the f a c t t h a t oxygen contents o f macropores can change i n a matter of hours d u r i n g an i n f i l t r a t i o n event, whereas micropores may remain suboxic r e g a r d l e s s o f flow c o n d i t i o n s (JUL) . In a d d i t i o n , micropores are l e s s s u s c e p t i b l e t o l e a c h i n g than macropores (2, 12-14). Because of these d i f f e r e n c e s , sample chemistry can v a r y widely from l o c a t i o n t o l o c a t i o n and from time t o time depending on the amount o f l i q u i d drawn from these two flow systems. Therefore, i t i s prudent t o c o n s i d e r u s i n g both unsaturated and f r e e drainage samplers i n a sampling program, depending on s i t e c h a r a c t e r i s t i c s . Table I. C r i t e r i a f o r S e l e c t i n g P o r e - L i q u i d Samplers 1. Required Sampling Depths 2. Required sampling Volumes 3. S o i l c h a r a c t e r i s t i c s 4. Chemistry and b i o l o g y o f the l i q u i d s t o be sampled 5. Moisture flow regimes 6. Required d u r a b i l i t y o f the samplers 7. Required r e l i a b i l i t y o f the samplers 8. Climate 9. I n s t a l l a t i o n requirements o f the samplers 10. O p e r a t i o n a l requirements o f the samplers 11. Commercial a v a i l a b i l i t y 12· Costs Specific guidelines f o r d e s i g n i n g vadose zone monitoring programs have been d i s c u s s e d (1-2, 7-9, 15-20) . S u c t i o n Samplers

(Unsaturated/Saturated sampling)

Table I I presents s u c t i o n samplers and some o f t h e i r o p e r a t i o n a l c o n s t r a i n t s . In g e n e r a l , a s u c t i o n sampler


19. DORRANCE ET AL.

Table II: Sampler Type

In Situ Pore-Liquid Samplers for Vadose Zone

Suction Sampler Summary Porous Section Material

Max. Pore Size (um)

Air Entry Value (kPa)

(HB)* Opera(HL)* tional Suction Range (kPa)

Max. Operational Depth (m)


Commercially-Available Suction Samplers Vacuum Lysimeters

Ceramic PTFE Stainless Steel

1.2-3.0 15-30 7

>100 5-10 20

HL HB HL

< 60-80

Compendium of In Situ Pore-Liquid Samplers for Vadose Zone

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