Groundwater Residue Sampling Design - American Chemical Society

Chapter 17

Experiences and Knowledge Gained from Vadose Zone Sampling 1

1

2

J. L Starr , J. J. Meisinger , and T. B. Parkin Downloaded by NORTH CAROLINA STATE UNIV on August 6, 2012 | http://pubs.acs.org Publication Date: June 20, 1991 | doi: 10.1021/bk-1991-0465.ch017

1

Environmental Chemistry Laboratory, Agricultural Research Service, U.S. Department of Agriculture, BARC-West, Beltsville, MD 20705-2350 National Soil Tilth Laboratory, Agricultural Research Service, U.S. Department of Agriculture, 2150 Pammel Drive, Ames, IA 50011 2

Vadose zone sampling offers an opportunity for assessing the impact on groundwater quality of chemicals applied at the land surface. Many interacting factors control the fate of chemicals in the f i e l d cause major sampling problems even for experienced researchers. Underlying any sampling program i s the absolute need to clearly define the study s objectives. The sampling procedure should then be developed with a clear conceptual view of the physical, chemical, and biological processes that affect the fate of the chemical (s) under investigation. Basic questions regarding the s p a t i a l , temporal, and s t a t i s t i c a l distributions of specific parameters must also be addressed in developing an efficient sampling plan. There i s no "best sampling method" for all situations, rather, there are several techniques with attendant advantages and disadvantages. An efficient sampling plan considers: the underlying processes; s p a t i a l , temporal, and s t a t i s t i c a l distributions of important parameters; and limited resources to answer the study's objectives. '

I n c r e a s i n g concern about the presence o f n u t r i e n t s , pesticides, and other chemicals i n shallow and deep groundwater, along with the d i f f i c u l t y i n q u a n t i f y i n g and p r e d i c t i n g t h e i r transformations and movement, has r e c e n t l y l e d t o s e v e r a l symposia on the vadose zone (1-4). These symposia i l l u s t r a t e the c r o s s - d i s c i p l i n a r y nature o f the problems t h a t i n v e s t i g a t o r s face i n attempting t o chara c t e r i z e and q u a n t i f y the f a t e of chemicals i n the environment.

This chapter not subject to U.S. copyright Published 1991 American Chemical Society

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

GROUNDWATER RESIDUE SAMPLING DESIGN

280

Downloaded by NORTH CAROLINA STATE UNIV on August 6, 2012 | http://pubs.acs.org Publication Date: June 20, 1991 | doi: 10.1021/bk-1991-0465.ch017

Vadose Zone Dynamics The vadose zone represents the three-dimensional geo l o g i c a l p r o f i l e above the ground water t a b l e , extending t o or through (depending on the chosen d e f i n i t i o n ) t h e b i o l o g i c a l l y a c t i v e l a y e r a t the s o i l s u r f a c e . Depending on the three-dimensional s p a t i a l d i s t r i b u t i o n i n h y d r a u l i c c o n d u c t i v i t i e s , p o r t i o n s of the vadose zone may become s a t u r a t e d f o r v a r y i n g lengths of time f o l l o w i n g p r e c i p i t a t i o n o r i r r i g a t i o n events. The p a r t i c u l a r problems and o p p o r t u n i t i e s a s s o c i a t e d with sampling the vadose zone a r e inherent i n i t s nature. The depth, h y d r a u l i c p r o p e r t i e s , and the nature o f t h e chemical transformations i n the vadose zone combine with the h y d r o l o g i e recharge c y c l e t o determine t h e extent t o which chemicals a p p l i e d t o the land s u r f a c e impact groundwater resources. Under c o n d i t i o n s of one-dimensional downward flow, a deep vadose zone can p o t e n t i a l l y s t o r e a l a r g e r q u a n t i t y of a p a r t i c u l a r s o l u t e than a shallow vadose zone, and t h e r e f o r e can supply s o l u t e s t o groundwater f o r many years a f t e r t h e s o l u t e i s no longer a p p l i e d t o t h e s o i l s u r f a c e . For non-conservative chemicals t h i s vadose zone c o n d i t i o n may provide the time-space needed f o r s i g n i f i c a n t s o l u t e a t t e n u a t i o n t o occur before i t reaches the groundwater. In c o n t r a s t , s o l u t e s i n a shallow vadose zone may move q u i t e r a p i d l y t o the groundwater, but t h e p o t e n t i a l f o r r e s i d u a l e f f e c t s i s reduced. (Threedimensional flow e f f e c t s w i l l be mentioned l a t e r i n t h i s chapter). The concept of a r e p r e s e n t a t i v e elementary volume (REV) of a porous m a t e r i a l can provide a u s e f u l framework f o r e s t i m a t i n g t h e volume of i n d i v i d u a l samples needed t o p r o p e r l y c h a r a c t e r i z e vadose zone parameters (5-8). A REV i s t h e sample volume, from a given domain, f o r which i n d i v i d u a l measurements of a given parameter (P) approach a s t a t i s t i c a l constant, independent of the sample volume. At very small sample volumes, the range o f Ρ v a l u e s w i l l f l u c t u a t e g r e a t l y due t o extreme e f f e c t s t h a t can occur a t the m i c r o s c o p i c l e v e l . As the sample volume i n c r e a s e s , the m i c r o s c o p i c e f f e c t s decrease and macroscopic e f f e c t s of t h e domain i n c r e a s i n g l y dominate the v a r i a t i o n o f P. The REV i s the sample volume a t which the parameter v a r i a t i o n i s p r i m a r i l y c o n t r o l l e d by macroscopic c o n d i t i o n s . The REV may be q u i t e d i f f e r e n t f o r d i f f e r e n t measured param e t e r s , and may vary with time due t o changing c o n d i t i o n s t h a t impact on t h a t parameter. Many f a c t o r s (e.g., vadose zone p r o p e r t i e s and c u l t u r a l treatments) a f f e c t the f a t e (transformations, adsorpt i o n - d e s o r p t i o n , and movement) o f chemicals i n t h e vadose zone (9,10) . A p a r t i a l l i s t i n g of dynamic and s t a t i c vadose zone p r o p e r t i e s and model parameters i s presented i n Table 1 (compare Table 2-2 i n Jury ( 9 ) ) . Static properties are a s s o c i a t e d with t h e i r l o c a t i o n i n the vadose zone. Mea surements of the static p r o p e r t i e s near t h e land s u r f a c e


17. STARR ET AL.

281

Vadose Zone Sampling


may change i n time due t o b i o l o g i c a l a c t i v i t y , temperature, t i l l a g e , s o i l t r a f f i c , e t c . The dynamic vadose zone f a c t o r s are l a r g e l y a s s o c i a t e d with the water phase (and are a f f e c t e d by f a c t o r s t h a t move water i n the vadose zone), and b i o l o g i c a l phenomena. Many of the f a c t o r s are interdependent which can r e s u l t i n l a r g e s p a t i a l and temporal v a r i a t i o n . T h i s interdependency o f t e n g i v e s r i s e t o frequency d i s t r i b u t i o n s t h a t are h i g h l y skewed and c o e f f i c i e n t s of v a r i a t i o n i n excess of 100% (11) . T a b l e 1. P a r t i a l l i s t of vadose zone p r o p e r t i e s and model parameters that can a f f e c t s o l u t e con centrations DYNAMIC Water content, θ Evapotranspiration Biological activity Solute c o n c e n t r a t i o n Solute v e l o c i t y and dispersion coefficients Structure Hydraulic conductivity, K

STATIC*

Q

Texture Bulk d e n s i t y Porosity S o i l water c h a r a c t e r i s t i c Adsorption parameters Cation exchange c a p a c i t y Saturated h y d r a u l i c conductivity, K s

*or "Semi-Static" as most of these may be a f f e c t e d by biological activity, cultural practices, etc. Sampling the Vadose Zone Sampling the vadose zone i s r e q u i r e d i n order t o q u a n t i f y and p r e d i c t the r a t e s of r e a c t i o n , transformation, and movement of chemicals downward t o groundwater, and l a t e r a l l y along i n c l i n e d t e x t u r a l l a y e r s and i n p h r e a t i c l a y e r s t o drainage d i t c h e s , streams, e t c . Proper development of a vadose zone sampling scheme r e q u i r e s knowledge of the p r i n c i p a l f a c t o r s c o n t r o l l i n g the f a t e of the chemicals i n the vadose zone, t h e i r interdependencies, and the probable frequency d i s t r i b u t i o n s of the observations. An i n i t i a l estimate of these items may be a v a i l a b l e i n the l i t e r a t u r e , but a p r e l i m i n a r y sampling survey w i l l provide the best i n f o r m a t i o n f o r determining the best experimental meth odology a t a s p e c i f i c s i t e . A wide v a r i e t y of techniques e x i s t s f o r sampling the l i q u i d and s o l i d phases of the vadose zone. Many of t h e i r advantages and disadvantages are presented elsewhere i n t h i s book as w e l l as i n recent l i t e r a t u r e reviews (1,12-14). S e v e r a l of these sampling methods are shown i n Table 2. I t i s important t o recognize t h a t d i f f e r e n t vadose zone sampling methods may a l s o measure d i f f e r e n t s o i l p r o p e r t i e s (e.g., s o l u t e or s o i l mass c o n c e n t r a t i o n s ) , and r e f l e c t d i f f e r e n t time increments (e.g., at a given time or averaged across time), and zones of i n f l u e n c e (e.g., particle s u r f a c e s , micro t o macro pores) · Because d i f f e r e n t sampling methods sample d i f f e r e n t e n t i t i e s (Table 2), i t i s un-


282


derstandable t h a t data produced by d i f f e r e n t r e s e a r c h s t u d i e s w i l l o f t e n produce seemingly c o n f l i c t i n g i n t e r p r e t a t i o n s . T h i s i s p a r t i c u l a r l y t r u e i n the e a r l y stages of a r e s e a r c h t h r u s t before a u n i f i e d understanding of the phenomena emerges. Table 2. C h a r a c t e r i s t i c s of sampling methods vadose zone, i n c l u d i n g s a t u r a t e d l a y e r s


Method

Samples*

S u c t i o n Cups

Time Step * P o r e s 1

c

for

the

Depth

(m)

0.1-3

C

Ρ

b, c

T i l e Lines

C, M

A,Ε

b,c

5

C, M FP

Ρ Ε

a b,c

0-1+ 0-2+

Lysimeter

Shallow

Wells

Deep Wells Excavation: Augers, cores Dye t r a c e r s a b c

C: Pore Water Concentration; M: Mass; PP: Flow Path. Ρ: P o i n t i n time; A: Average flow; E: By Event. Diameters (nm) a: 1000.

Other chapters i n t h i s book have emphasized t h a t un d e r l y i n g any sampling program i s the a b s o l u t e need t o c l e a r l y d e f i n e the study's o b j e c t i v e s , and t o i d e n t i f y the s p e c i f i c vadose zone information t h a t i s needed t o achieve the o b j e c t i v e s . Even though there i s a tendency t o use only one or two p e r s o n a l l y " t r i e d and t r u e " methods f o r a l l i n v e s t i g a t i o n s , the m u l t i p l i c i t y of methods a v a i l a b l e f o r sampling the vadose zone (12-14), suggests t h a t t h e r e i s no "best" sampling method f o r a l l s i t u a t i o n s . A survey of a watershed f o r the mass of chemical r e s i d i n g i n the vadose zone a t a p o i n t i n time n e c e s s i t a t e s a d i f f e r e n t approach than t h a t needed t o monitor the changes i n chemical c o n c e n t r a t i o n i n the mobile water phase as a f u n c t i o n of depth over some time i n t e r v a l . For example, a watershed survey might be accomplished using s o i l core data w h i l e monitoring s o l u t e chemical concentrations might be ac complished with s u c t i o n cups or shallow w e l l s . Several approaches are u s u a l l y needed t o c h a r a c t e r i z e and q u a n t i f y the processes c o n t r o l l i n g the f a t e and movement of a vadose zone chemical t o the groundwater (9). E v a l u a t i n g Sampling

Techniques

In an attempt t o determine the REV f o r s e v e r a l s o i l pa rameters from s o i l cores, e.g., d e n i t r i f i c a t i o n (15) along


17. STARR ET AL

283

Vadose Zone Sampling

w i t h s e v e r a l other chemical and p h y s i c a l p r o p e r t i e s (16), we conducted four vadose zone sampling experiments with s i x sample volumes (38, 58, 149, 216, 366, and 8770 cm ) from the s u r f a c e h o r i z o n . In the f i r s t experiment, 36 r e p l i c a t e samples were taken t o a depth of 16 cm from w i t h i n a 1.2 χ 1.8 m area. F i v e s o i l cores were each taken i n s i d e each 8770 cm sample (0.2 by 0.3 m template) . In the f i r s t experiment, the l a r g e r e c t a n g u l a r samplers were p l a c e d d i r e c t l y adjacent so t h a t a l l the s o i l i n the sampling area was removed from the sampling area. Hence the weighted mean of each measured parameter was equal t o the p o p u l a t i o n mean. Due t o the d i s p r o p o r t i o n a t e volume of the r e c t a n g u l a r sample ( s i z e 6), the p o p u l a t i o n mean was a l s o approximately the same as the mean of s i z e 6. F i g u r e 1 shows the chemical mass ( r e l a t i v e t o the p o p u l a t i o n mean, dashed l i n e ) f o r the f i r s t f i v e sample s i z e s f o r N0 -N and ortho-P, with 90% confidence l i m i t s (solid lines). Land's method t o compute the confidence i n t e r v a l s was used because i t provides exact confidence i n t e r v a l s f o r lognormal d i s t r i b u t i o n s (17,18). The amount of skewness f o r each sample volume mean may be judged by the amount of displacement of the mean from the midpoint between L a n d s 90% confidence i n t e r v a l s . Although the r e l a t i v e v a r i a t i o n f o r Ν was n e a r l y twice t h a t f o r P, the o v e r l a p p i n g confidence i n t e r v a l s f o r the d i f f e r e n t sample and p o p u l a t i o n means i n d i c a t e t h a t the REV f o r both pa rameters may be somewhat smaller than the s m a l l e s t sample volume (38 cm ) . 3


3

3

1

3

Bootstrapping. Bootstrapping provides another way t o estimate the REV f o r t h i s data s e t because i t p r o v i d e s a way t o c h a r a c t e r i z e the range of v a r i a n c e s a s s o c i a t e d with the different sample sizes. Bootstrapping is a computer-intensive random resampling technique (19) t h a t r e q u i r e s no assumptions regarding the u n d e r l y i n g p o p u l a t i o n d i s t r i b u t i o n (19-22). For t h i s method, data subsets are randomly drawn from the o r i g i n a l data s e t with the number of observations (n) f o r each subset being the same as i n the o r i g i n a l data s e t . As an example of t h i s method, 1000 N0 -N v a r i a n c e s , a s s o c i a t e d with the 1000 b o o t s t r a p means, are p l o t t e d f o r each sample volume (Figure 2). Based on o v e r l a p p i n g 95% confidence i n t e r v a l s , the v a r i a n c e s are not s t a t i s t i c a l l y d i f f e r e n t f o r the f i r s t f i v e sample s i z e s . However, the 50% decrease i n v a r i a n c e from the f i r s t t o second sample s i z e may i n d i c a t e t h a t the REV i s b e t t e r represented by sample volumes c l o s e r t o 58 cm , r a t h e r than "somewhat < 38 cm " as suggested above. 3

3

3

Random subsampling. Random subsampling i s another computer-intensive random resampling technique t h a t can be used t o determine the t o t a l mass of s o i l r e q u i r e d t o best estimate the p o p u l a t i o n mean f o r v a r i a b l e s with a lognormal distribution. Random subsampling d i f f e r s from boot s t r a p p i n g i n t h a t the number of observations randomly drawn


284



ο Ο

ο Ο

ο CL

Figure 1. Mean r e l a t i v e nitrate-N and ortho-P c o n c e n t r a t i o n s ( s o l i d squares) with 90% confidence window ( s o l i d l i n e ) vs l o g [ s a m p l e volume (cm )]. The dashed l i n e r e p r e s e n t s t h e p o p u l a t i o n mean. 3

10

500

UJ ο ζ

Groundwater Residue Sampling Design - American Chemical Society

Recommend Documents