Journal of Environmental Protection, 2011, 2, 629-638
10.4236/jep.2011.25072 Published Online July 2011 (
Copyright © 2011 SciRes. JEP
Physicochemical Characterization of Sediment in
Northwest Arkansas Streams
Chris W. Rogers1, Andrew N. Sharpley1, Brian E. Haggard2, J. Thad Scott1, Bodie M. Drake1
1Department Crop, Soil, and Environmental Sciences, University of Arkansas Division of Agriculture, Fayetteville, USA; 2Arkansas
Water Resources Center, University of Arkansas Division of Agriculture, Fayetteville, USA.
Received March 29th, 2011; revised May 2nd, 2011; accepted June 10th, 2011.
Eutrophication of surface waters is a critical concern in regions around the world facing nutrient surpluses as a result
of confined animal feeding operations (CAFOs) and subsequent land application of manures. While large amounts of
research exist on the transport of nutrient enriched runoff from fields to surface waters less information is available on
in-stream processes controlling the transport of P in-stream. Thus, information is needed on the role of stream sedi-
ments in regulating transient phosphorus (P) to better understand the relationship between nutrient inputs and water
quality. Fine-sized sediments (< 2-mm) regulate P via sorption and burial, while algae attached to larger-sediments (>
2-mm) consume and store P. From fine-sized sediment a modified P saturation ratio (PSRmod), related to the sediments
ability to bind P and determined from Mehlich-3 extracted nutrients, has been correlated to in-stream dissolved reac-
tive P (DRP) concentrations. The objectives of this study were to determine the relative size distribution of total- and
fine-sized sediment (sand, silt clay) fractions among streams, determine the optimum sample number needed to char-
acterize Mehlich-3 P (M3P) and PSRmod, and finally determine the applicability of PSRmod as an indicator of stream
water column DRP concentrations. Stream sediments were sampled from the 0- to 3-cm depth from stream reaches
ranging from (2575 m) in August, 2008 for characterization along with water samples collected from the thalweg for
DRP concentration determination. Additional water column samples were collected along with fine-sized sedi- ment
chemical properties in February, May, and June 2009. The distribution of sediment size classes was statisti- cally simi-
lar, with 2- to 20- and 20- to 75-mm sized sediment dominating. Fine-sized sediment (< 2 mm) contributed 9% to 18%
of total-sediment and was comprised primarily of sand. Sampled stream M3P and PSRmod were determined to typically
be sufficiently characterized by a sample scheme utilizing three samples points. Modified P saturation ratio of < 2-mm
sediment was highly correlated to DRP levels across sampling dates (r = 0.86), suggesting PSRmod has the potential to
be used as an indicator of the ability of stream sediments to enrich stream water with P. Thus, fine-sized sediment nu-
trient concentrations appear to be key regulators of water column P concentrations.
Keywords: Stream, Phosphorus, Nutrient Enrichment, Sediment
1. Introduction
Accelerated eutrophication of freshwaters is identified as
the leading impairment of water quality in the United
States [1]. In freshwater lakes, phosphorus (P) is linked
to increased algal productivity, as it is the most common
limiting nutrient for algal growth [2,3]; however, controls
in stream systems can be more complex [4]. It is under-
stood that stream sediment characteristics influence P
types and amounts transported from the landscape to
lakes and reservoirs. Within the fluvial system, sediments
act as either sinks or sources of P and thus, may be influ-
ential in determining the time frame over which changes
occur in watersheds after management strategies have
been implemented [5].
Sediment size fractions have important impacts on de-
termining dominate processes controlling P-forms in
streams. Phosphorus interaction with fine-sized sediments
(<2-mm) are typically regulated by abiotic reactions, and
with increasing size, biological control associated with
attached algal growth becomes the dominate P uptake
mechanism [6]. Work in the United Kingdom reported P
release from algal-biofilms on large-sized sediments (>
20-mm) to stream water was greater than release from
fine sediments (<2-mm) [7]. Howev er, Lottig and Stanley
[6] reported that fine-sediments (< 2-mm) had a greater
capacity to adsorb P than larger sediments and thus, had
Physic oc hemical Characterization of Sediment in Northwest Arkansas Streams
a greater capacity to act as buffers of P entering during
episodic rainfall-runoff events.
We determined the relative size distributions of bed
sediments of five streams in the Upper Illinois River
Watershed (UIRW) in Northwest Arkansas during base-
flow conditions, landuse within the watershed draining
into them, and sediment chemical properties using rou-
tine soil extractions. Specifically, we determined Meh-
lich-3 P and modified P saturation ratio (PSRmod), a pa-
rameter calculated from Mehlich-3 extractable nutrient
concentrations of M3P and P sorbing elements [8,9].
Modified P saturation ratio was only recently reported by
Haggard et al. [9] during an experiment sampling stream
sediments from May to June, 2006 and thus, warrants
further investigation to determine its applicability.
Specific objectives of the study were to determine the
distribution of sediment proportions across streams and
determine if relative proportions were similar among
sampled streams, and as research constraints often pose
limitations on the number of sediment samples which can
be collected, we determined the optimal number of sam-
ples needed to characterize both M3P and PSRmod. Fi-
nally, we investigated the applicability of PSRmod as an
indicator of stream DRP concentrations across various
streams and sampling dates.
2. Methods
2.1. Study Site Description
Northwest Arkansas is characterized by gently sloping
hills and karst limestone geology, and was once domi-
nated by oak (Quercus spp.) - hickory (Carya spp.) for-
ests with areas of tallgrass prairies [10]. Within the re-
gion, these prairies have largely been converted to pas-
ture and hay fields with poultry-beef cattle production
systems dominating agricultural production. Furthermore,
a large portion of the soils within the reg ion situated atop
the underlying karst limestone geology are stony, shal-
low, and occur on steep slopes, which can lead to rapid
surface runoff and groundwater leaching [11].
Agricultural land use in Northwest Arkansas is often
cited as a leading contributor to in creased inpu ts of P into
the waters of the region [12]. This is partly due to the
rapid increase in population growth over the last 20 years
and the area’s large number of poultry prod uction opera-
tions and associated litter, which is often spread on local
pastures [13,14]. With continued application of litter at
rates to meet forage nitrogen (N) requirements, soil P can
accumulate to levels that increase the risk of P enrich-
ment of runoff [13]. Urban areas in the region are also
important sources of P, as they have large amounts of
impervious surfaces, inputs from lawns, and waste water
treatment plants [15]. Nutrient enrichment of surface
waters from agricultural and urban sources is a world-
wide phenomenon and similar issues arise in areas as
diverse as Arkansas, Denmark, and Ireland [16]. These
increases in CAFOs can often lead to a localized surplus
of manure, and in time, can lead to the increased suscep-
tibility of P runoff from highly P enriched surface soils
Five streams draining into the Illinois River in North-
west Arkansas were selected with agricultural, forested,
and urban land uses present within each. Subwatersheds
were delineated by sampling sites using a digital eleva-
tion model and land use/land cover data in ArcGIS 9.2
[17-19]. The delineated subwatersheds were determined
using the ArcHydro tool within ArcGIS and proportion
land use was calculated as percentages of total land area
within each subwatershed.
2.2. Sample Collection and Analysis Techniques
A representative reach (riffle and pool) was identified at
each of the 5 streams in August, 2008. Reaches (25 - 75
m) were measured at equally spaced intervals with 7 to
10 transects collected to determine the relative substrate
composition of the streams. As sediment distribution and
characterization was a primary objective at the August,
2008 sample date, each individual transect was collected
separately. Transect width was measured and stream ve-
locity measured using a Flo-Mate 2000 (Marsh-McBirney,
Inc., Frederick, MD) at equally spaced points across the
individual transects upstream at each sampling site dur-
ing baseflow conditions. Width and velocities were used
to calculate flow rate and average velocity across each
transect within the reach.
At each site, a stream water sample was collected from
the thalweg at the time of sediment sampling. A water
subsample was filtered (0.45-µm), acidified to pH 2 (HCl)
in-field, and transported to the laboratory and stored at
4˚C until analysis. An unfiltered water sample was also
acidified (pH 2) and stored at 4˚C until analysis. Filtered
samples were analyzed for DRP using the automated
ascorbic acid method on a Skalar San Plus Wet Chemi-
stry Autoanalyzer (Skalar, the Netherlands) [20]. The
total P (TP) concentration of unfiltered samples was de-
termined after digestion via an alkaline persulphate
method [21 ,22].
A 2 L sample of total-sediment was collected from a
0- to 3-cm depth at five locations across each transect
with a spade and composited to create a representative
sample. Samples from the transect were sieved to size
classes of > 75, 75 to 20, 20 to 2, and < 2-mm within 24
h after sampling. A < 2-mm sample of sediment was col-
lected at 5 locations along each transect and composited
for subsequent particle size analysis and a sub-sample air
dried prior to determination of Mehlich-3 extractable
opyright © 2011 SciRes. JEP
Physic oc hemical Characterization of Sediment in Northwest Arkansas Streams631
nutrients. Each size fraction of sediment was air dried.
After drying, size classes were measured using a standard
water displacement procedure, where water (6 L) was
added to a container and the volume of water displaced
measured when each size class was added. Total displa-
cement of all size classes was determined along each
transect and each size class divided by this total to deter-
mine relative proportion of each size class within the
stream transect.
Particle-size distribution of the fine-fraction sediment
(< 2-mm) was determined by the hydrometer method
[23]. Sediments were added at 50 g dry weight to a 1 L
cylin- der with 50 mL of sodium hexametaphosphate and
brought to volume. Three hydrometer readings were
taken and averag ed at the 40 s mark, and one 2 h reading
was taken. These reading were then used to calculate the
relative proportion of sand (2 - 0.05-mm), silt (0.05 -
0.002-mm), and clay (< 0.002-mm) at each transect
within each stream. Mehlich-3 extractable P of the
fine-fraction was determined by shaking 1 g air-dried
sediment with 10 mL mixture of 0.2 M CH3COOH, 0.25
M NH4NO3, 0.015 M NH4F, 0.013 M HNO3, and 0.001
M EDTA end-over-end for 5 min [8]. Mehlich-3 extracts
were centrifuged (2 500 g for 10 min) and filtered (0.45
µm) prior to determining P, Fe, Mg, Mn, and Ca [8]. De-
termination of the mini- mum sample number necessary
to describe Mehlich-3 P and PSRmod within the sample
stream reaches were determined based on individual
transect determination of Mehlich-3 P and PSRmod from
the August, 2008 sample date.
To investigate the relationship between PSRmod and
DRP concentrations in stream water, a composite fine
sized sediment (< 2-mm) sample from the previously
identified transects and water column samples for DRP
were conducted three additional times (February, May,
and June 2009). As stream sediments had been charac-
terized by the initial August, 2008 sampling, the rela-
tionship between fine-sized sediment chemical properties
across sampling dates was determined on a single com-
posite sample from each stream in August, 2008, Febr-
uary, May, and June 2009.
2.3. Modified Phosphorus Saturation Ratio
Mehlich-3 P has been used as an indicator to estimate the
potential for soil P release to runoff [24,25]. In Sims et al.
[26], P saturation ratio (PSR) was determined from Me-
hilich-3 extractable elements as PSR = [M3P / M3Al +
M3 Fe]. This ratio includes P-reactive trace elements and
improves upon the relationship between soil test P mea-
sures and subsequent DRP concentrations of surface run-
off, as increases in these trace elements decrease the
movement of P into the water column [26]. For calcare-
ous soils, inclusion of M3Ca and M3Mg improved the
relationship between PSR and runoff DRP [27]. Haggard
et al. [9] modified the P saturation ratio (PSRmod) to
[M3P / M3Fe + M3Mg + M3Mn] for calcareous str eams.
Modified P saturation ratio was found to be statistically
more closely correlated to in-stream DRP concentrations
(r = 0.71) than M3P alone (r = 0.50) in Haggard et al. [9].
As only a limited sampling p eriod was used to determine
PSRmod [9], the current research will further investigate
the effectiveness of the parameter in predicting DRP
concentrations in stream water at different times and lo-
2.4. Statistical Analysis
Linear regressions were performed in SigmaPlot with
significance levels of α < 0.05. Regressions of large-
sized sediment and DRP are constructed from compo-
sited sediment values at individual streams and sampled
DRP concentrations from the August, 2008 sampling
date. Composited fine-sized sediment samples from four
sample dates (August 2008, February, May, and June
2009) are used for the regressions focusing on the fine-
sized sediment chemical parameters of Mehlich-3 P and
PSRmod. Relationship strength between parameters within
the text is discussed in terms of correlation coefficients
(r). When regression models are presented in figures co-
efficients of determination (R2) are reported.
To determine if differences in sediment distribution
existed among streams, a one-way analysis of variance
(ANOVA) was performed in SAS 9.2. Transects of the
August, 2008 sampling date were used as replicates and
streams treated as fixed factors and the four size classes
and three < 2-mm size classes tested separately. When
stream effect was significant, means were separated us-
ing Fisher’s protected least significant difference (LSD).
Differences in sample number (n) for this analysis are
based on the number of equally spaced transects mea-
sured at individual sites.
For the samples collected in August, 2008 at individ-
ual transects, M3P and PSRmod were tested in a multiple
step procedure to determine the minimum number of
samples necessary to quantify M3P and PSRmod. The
procedure for this analysis was conducted as follows,
first the sample means were calculated from the entire set
of sampled transects at each site and treated as the “true”
mean. Given the restriction of end samples always being
taken for each possible sample size, a complete enumera-
tion of all possible samples for a given size (e.g., sample
size 5) were generated, and 95% confidence intervals
(C.I.) were constructed using a standard T-statistic. The
number and percentage of samples in which the confi-
dence interval contained the “true” mean was calculated.
The smallest sample size for which the percentage of
samples within the 95 % confidence in terval was equal to
Copyright © 2011 SciRes. JEP
Physic oc hemical Characterization of Sediment in Northwest Arkansas Streams
Copyright © 2011 SciRes. JEP
or greater than 95% was deemed sufficient to be used for
sampling of that parameter. In this analysis n refers to
individual transects from each site.
were tested separately. Parameters which had a signifi-
cant stream effect were subsequently separated using
Fisher’s protected least significant difference (LSD).
Composite samples were used for this analysis an d there-
fore, n in this analysis refers to the sampling dates of
August 2008, February, May, and June 2009.
A second ANOVA was performed for samples colle-
cted at the four sampling dates. Both water column P
concentrations and chemical parameters associated with
the fine-sized sediment were compared across streams. In
this analysis, an overall composite from the August, 2008
sampling date was used along with the additional sam-
pling dates of February, May, and June 2009. These sam-
pling dates constitute four replications and within the
analysis, stream was treated as a fixed factor and DRP,
pH, M3P, M3Ca, M3Fe, M3Mg, M3Mn, and PSRmod
3. Results and Discussion
3.1. Land Use
Stream sites within the region represented varying land
uses and within each site, agricultural, forested, and ur-
ban activities were present (Figure 1). Mud Creek Tribu-
Figure 1. Map of delineated research site drainage basins with land use land classification (agriculture, for est, urban) for five
streams in the Upper Illinois River Watershed, AR.
Physic oc hemical Characterization of Sediment in Northwest Arkansas Streams633
tary had the most urbanized subwatershed of those stud-
ied with 68% of the drainage area dominated by this land
use and only 7% composed of agricultural land (Table 1).
In contrast, Wildcat Creek was nearly 70% agriculture
and 4% urban. Chamber Springs with 61% forest was the
only primarily forested landscape; however, the water-
shed is also comprised of 38% agriculture much of which
is directly adjacent to the stream (Table 1). Finally,
Moore’s Creek was comprised mainly of agriculture and
forested land with 56% and 39% of the total land area in
the land uses respectively.
3.2. Stream Flow and Velocity (August, 2008)
Average stream velocity at the time of stream characte-
rization (August, 2008) w as variable across sites, ranging
from 0.06 to 0.26 m·s–1 (Table 2). Decreased sediment/
water interaction time in higher velocity streams has been
cited as decreasing P uptake in streams, and since these
streams were all at baseflow conditions it appears likely
P retention time within streams will be i mpacted by ve lo-
city and that faster moving streams will have less time to
interact with transient P [28]. The current study had
slightly higher velocities overall than those reported by
D’Angelo et al. [28] from North Carolina streams (0.04 -
0.17 m·s–1). As all subsequent sampling times (February,
May, and June 2009) were conducted during baseflow
conditions, it is likely that sediment water interactions
Table 1. Percentage of watershed in agriculture, forest, and
urban land use categories for five selected streams in the
Upper Illinois River Watershed, AR.
Stream Urban (%)Agriculture
(%) Forest
Chamber Springs 1 38 61*
Little Wildcat Creek 19 61 20
Mud Creek Tributary 68 7 25
Moore’s Creek 6 56 39
Wildcat Creek 4 70 26
*Percentages in italics represent the major land use with a give stream wa-
Table 2. Water column parameters including Dissolved
Reactive P (DRP) and Total P (TP) concentrations for five
selected streams in the Upper Illinois River Watershed, AR
in August, 2008.
Stream Velocity (m3·s–1) Flow rate (m3·s–1)
Chamber Springs 0.20 0.08
Little Wildcat 0.26 0.16
Moore’s Creek 0.12 0.20
Mud Creek Tributary 0.06 0.02
Wildcat Creek 0.20 0.19
in these streams are decreased even more during high
velocity events such as during high rainfall, and nutrient
transport to overlyin g waterbodies and that P transport is
elevated at those times.
Flow rates also varied across streams and ranged from
0.02 to 0.20 m3·s–1 (Table 2). Thus, varying transient
storage times across sites occurred, which may lead to
differences in P uptake due to water-sediment residence
time differences. For example, Mud Creek Tributary is
likely to have flashy events where water levels rise rap-
idly due to the high percentage of urban land within the
watershed and thus, during these events markedly de-
creased interaction with stream bed sediment is likely to
3.3. Sediment Distributions (August, 2008)
The relative size distribution of total sediment classes
was determined in August, 2008 and was similar across
sites with no statistical differences to report. Based on
volumetric displacement, the > 20-mm sized sediment
accounted for roughly 50% of bed sediment across all
sites (Table 3). Gainswin et al. [7] reported chlorophyll-
a concentrations of 2- to 20-mm size fraction sediment at
two sampled sites in the United Kingdom (6.4 and 8.6
mg·m-2, respectively) was appreciably lower than >20-
mm sediment (22.8 and 62.7 mg·m–2, respectively). Sedi-
ment of 2- to 20-mm cholorophyll-concentrations were
more similar to fine sediments, which had chlorophyll-
concentrations of 3.0 and 2.3 mg·m–2, respectively [7].
The intersection of these size classes likely represents an
important transition from abiotic to biotic dominance of
P reactions and transformations. Thus, bed sediments of
streams in the UIRW represent a system in which there
are large portions of sediment where P reactions are pre-
dominately biolog ically driven (> 20-mm) and large por-
tions which are abio tically driven (< 20-mm).
Further separating the size classes, the >75-mm sedi-
ments comprised the lowest percentage across sites,
ranging from 0.3% to 10.8% of bed sediment (Table 3).
The < 2-mm sediments were the second lowest, ranging
from 9% to 18% of streambed composition. However, <
2-mm sediments are likely to have the largest surface
area per unit weight of any sediment size class and thus,
greatest chemical reactivity. The 75- to 20-mm and 20-
to 2-mm classes represented roughly 80 to 90% of the
total bed material across sites and ranged from 35 to 48
and 36% to 46% of fluvial sediment, respectively. No
significant differences between streams for any size class
was observed; p-values ranged from 0.188 (20 to 2 mm)
to 0.534 (< 2 mm), thus, means were not separated by
Fisher’s Prote cted LSD (Table 3).
At the time of sampling, in-stream concentrations of
DRP decreased as the percentage of > 75-mm sediment
Copyright © 2011 SciRes. JEP
Physic oc hemical Characterization of Sediment in Northwest Arkansas Streams
Table 3. Sediment size classifications and < 2-mm particle size analysis for five selected streams in the Upper Illinois River
Watershed, AR in August, 2008.
Total Sediment (mm) < 2-mm particle size
Stream n > 75 75-20 20-2 < 2 Sand Silt Clay
% %
Chamber Springs 9 5.7 45.5 36.6 12.2 85.5b* 9.2a 5.3
Little Wildcat Creek 8 10.8 40.4 40.3 8.6 92.0ab 5.6ab 2.4
Moore’s Creek 9 0.3 35.0 46.2 17.9 90.9ab 3.8b 5.3
Mud Creek Tributary 10 12.2 40.4 36.2 11.2 94.8a 3.1b 2.1
Wildcat Creek 9 7.4 48.7 35.7 14.3 84.7b 9.8a 5.4
Standard Error (n = 10) 3.4 4.1 3.3 3.9 2.5 1.4 1.2
Standard Error (n = 9) 3.6 4.3 3.5 3.9 2.6 1.5 1.3
Standard Error (n = 8) 3.8 4.6 3.7 4.1 2.8 1.6 1.3
p-value 0.200** 0.220 0.188 0.534 0.048 0.008 0.145
increased (Figure 2). Total P concentrations also de-
creased when the percentage of > 75-mm sediment in-
creased (Figure 2). This is likely related to associated
periphyton growth on large sediments (> 20-mm) acting
as sinks of P during growth and uptake. However, upon
death and decomposition, algal biomass can become a
source of P to overlying waters [29].
3.4 Fine-Fraction Sediment (August, 2008)
The percentage of < 2-mm sized sediment was greatest at
Moore’s Creek and represented 18% of the total bed sub-
strate whereas, Little Wildcat Creek had the least fine-sized
sediment at 9%. Across streams, there was no significant
stream effect (p = 0.534) on the mean < 2-mm percentage
(Table 3). However, significant differences were appar-
ent in the < 2-mm size class fractions (sand, silt, clay).
Sand (0.05- to 2-mm) was the most predominant fraction
of < 2-mm sediment, comprising over 80% of the fine
sediment at each site (Table 3 ). Differences in the size of
the sand fraction existed among streams, with Mud Creek
Tributary having a significantly greater mean sand con-
tent than either Chamber Springs or Wildcat Creek (Ta-
ble 3). This fraction is often linked to highly available
P-fractions which are less tightly sorbed than on
clay-sized fractions, and thus, Mud Creek Tributary
likely has less P binding capacity during high input
events than Wildcat Creek or C hamber Springs.
The silt-sized fraction was also variable within sites
and across streams. Both Wildcat Creek and Chamber
Springs had a statistically greater mean silt-sized fraction
than Mud Creek Tributary (Table 3). The clay-fraction
was not statistically different among streams (p = 0.145),
but within sites ranged from 2.1% to 5.4% (Table 3).
While clay-fractions typically can hold the most P, this is
also contingent on the extent and duration of P inputs to
Figure 2. Relationship between Dissolved Reactive P con-
centration (DRP) Total P (TP) and average percent >
75-mm sediment at five streams in the Upper Illinois River
Watershed, AR.
opyright © 2011 SciRes. JEP
Physic oc hemical Characterization of Sediment in Northwest Arkansas Streams635
the stream. Similar to results from McDowell and Shar-
pley [30], our site with the greatest percentage of land
area as forest (Chamber Springs) had relatively high clay
content (5.3%) (Table 3) and the lowest M3P concentra-
tion, true mean, (Table 4) compared to the other sites.
This is likely due to a lack of P inputs to forested areas
and subsequent P runoff to the stream. Also, P held by
sand-sized particles is generally less tightly sorbed than
to clay-sized particles and is more easily released to wa-
ter [6]. While percent sand was statistically different
among sites, its general predominance (84.7% - 94.8 %),
and the fact that the mean clay content across sites was
statistically similar led to the lack of statistically signifi-
cant correlations between < 2-mm fractions and P con-
While the relative distribution of size classes in
streams plays a role in in-stream P transport, it appears
that the concentration of nutrients transported to the
stream and sorbed by fine-sized sediment can be quite
variable even in streams with similar stream bed compo-
sitions. This is likely due to the continued replenishment
of fine-sized sediment from the landscape in turn acting
as a renewal mechanism for P within these stream sites.
For example, Mud Creek Tributary, which drains a
highly urbanized area, experiences rapid influxes of wa-
ter during storm events, thus, sediments may be more
rapidly transported within th is stream than those draining
dominantly agricultural or forested areas. Subsequently,
these large loads can continually replenish its sand-sized
fraction (95%) and the remainder of the < 2-mm size
fraction of bed sediment with eroded soil [31]. In con-
trast, Moore’s Creek a predominately agricultural and
forested site is likely less influenced by rapid changes in
velocity and thus, water column sediment interactions are
a more dominant force, particularly as M3P concentra-
tions within this stream are higher than any other stream
sampled (Table 4).
3.5 Minimum Sample Number (August, 2008)
Previous work in the region focusing on P in fluvial
sediments, has sampled three transects within a stream
reach [9]. The current study collected 8 to 10 transect
sediment samples from each stream reach (Table 4). The
study reach lengths and total sample number were vari-
able across streams (26 - 69 m), because of differences in
accessibility and geomorphology. Thus, if we assume the
more rigorous sampling protocol of the current study
precisely characterized Mehlich-3 extractable nutrient
concentrations of the benthic sediments using these mul-
tiple transects then we could evaluate the minimum
number of transects needed to characterize M3P. Based
on the five sampled streams, the minimum number of
transects needed for 95% coverage of the 95% confi-
dence interval of the “true” mean of M3P at the sites
sampled was generally three (Table 4). The exception
was at Wildcat Creek where 4 samples were needed. For
PSRmod the trend was similar except Wildcat Creek re-
quired 5 samples to cover the confidence interval. The
issue at Wildcat Creek is that the site had a larger amount
of variation within as illustrated by the large standard
errors (Table 4). Thus, when sites are sampled, careful
attention to the insite variability is likely a clue as to
whether a set of 3 samples is sufficient to adequately
describe the sampled stream reach, and if large variation
exist a larger sample number would be recommended.
3.6. Mehlich-3 Content of Fluvial Sediments
(August 2008, February, May, and June
The Mehlich-3 P concentrations of < 2-mm sediments
ranged from 13 to 39 mg·P·kg–1 (Table 5). It appears that
sediment M3P was highly correlated to DRP (r = 0.86),
with increased streambed M3P content lead ing to greater
stream DRP concentrations (Figure 3). This relationship
is stronger than previously reported for this region by
Haggard et al. [9; r = 0.50], but with a similar slope
(0.0016 compared to the prior 0.0022). However, as our
relationship between M3P and stream DRP is driven by
one high-P site (Moore’s Creek), we further investigated
PSRmod as a more accurate parameter for correlating
Table 4. Stream length and mi nimum number of samples needed to represent reach sediment Mehlich-3 P (M3P) and Modi-
fied P Saturation Ratio (PSRmod).
Mehlich-3 P PSRmod
Stream n Reach
Length “True
Mean” Standard
Error Mean Sample Size for
95% C.I. “True
Mean” Standard
Error Mean Sample
Size for 95% C.I.
m mg·P·kg–1 Sample number % Sample number
Chamber Springs 9 45 11.5 1.32 3 5.1 0.44 3
Little Wildcat Creek 8 26 16.9 1.93 3 5.0 0.43 3
Moore’s Creek 9 41 37.0 1.96 3 7.9 0.35 3
Mud Creek Tributary 10 69 13.7 1.59 3 3.3 0.36 3
Wildcat Creek 9 38 15.6 2.17 4 4.0 0.66 5
Copyright © 2011 SciRes. JEP
Physic oc hemical Characterization of Sediment in Northwest Arkansas Streams
Table 5. Mehlich-3 nutrients, Modified P Saturation Ratio (PSRmod), and pH of sediments from five selected streams in the
Upper Illinois River Watershed, AR from sampling dates in August 2008, February, May, and June 2009.
Stream n DRP pH M3P M3Ca M3Fe PSRmod
mg·P·L–1 mg·P·kg–1 %
Chamber Springs 4 0.410b* 7.5b 13c 1221c 156b 4.7b
Little Wildcat Creek 4 0.029cd 7.7ab 15bc 825d 124b 4.8b
Moore’s Creek 4 0.074a 7.4b 39a 730d 358a 7.8a
Mud Creek Tributar y 4 0.021d 8.1a 13c 1878b 165b 2.7c
Wildcat Creek 4 0.035bc 8.1a 19b 2805a 162b 4.6b
Standard Error 0.004 0.2 2 113 17 0.4
p-value < 0.0001 0.02 < 0.0001 < 0.0001 < 0.0001 < 0.0001
Figure 3. Relationship between average Mehlich-3 P (M3P)
and Dissolved Reactive P concentrations (DRP) for five
streams in the Upper Illinois River Watershed, AR.
streambed chemical characteristics to overlying water
column DRP concentrations. Because sediment P avail-
ability is influenced by P-sorbing elements and the rela-
tionship between M3P and DRP concentrations is prob-
lematic due to the influence of Moore’s Creek, PSRmod
was utilized.
Calculated PSRmod ranged from 3% to 7.8% across
sites and may be a better indicator of P availability as it
considers Fe, Mg, and Mn concentrations, which influ-
ence P sorption and regulate P availability to the water
column (Table 5). Across sites, PSRmod for Moore’s
Creek sediment was the highest at 7.8 % and statistically
different from the remaining sites. Mud Creek Tributary
sediment was the lowest PSRmod (2.7 %) and was statis-
tically different from the other sites. Chamber Springs,
Little Wildcat Creek, and Wildcat Creek sediments have
similar PSRmod values (Table 4). When analyzed across
sampling dates, Modified P saturation ratio was highly
correlated to stream DRP concentrations (r =0.86) with a
slope of 0.0096 (Figure 4). The slope of the PSRmod and
stream DRP relationship in this study is approximately
two times that (0.004) found by Haggard et al., [9], indi-
cating DRP concentrations in some streams may be con-
trolled by fine-sized sediment nutrient concentrations and
may thus, have a greater impact on stream DRP concen-
Figure 4. Relationship between Modified P Satur ation Ratio
(PSRmod) of sediments and Dissolved Reactive P concentra-
tions (DRP) for five streams in the Upper Illinois River
Watershed, AR.
Based on the work of Lottig and Stanley [6] and our
results, it is probable that PSRmod decreases with increas-
ing amounts of larger-sized sediments and may be repre-
sentative of a shift in the buffering mechanisms from
abiotic to biotic control. Thus, in terms of P transforma-
tions between stream sediments and the water column, it
is important to consider the concentrations of trace ele-
ments as well as P. Differences in P-chemistry across
streams is likely due to other factors than simple sedi-
ment particle size distribution, as they are comparable in
their relative percentages (Tab l e 3 ). Other factors such as
P input source, biological uptake, clay mineralogy, sedi-
ment P concentrations, differences in < 2-mm sediment
and P inputs to the stream from the landscape will be
important in determining P transformations and transport
in these streams.
4. Conclusions
It is apparent that many variables contribute to determin-
ing the DRP concentration of stream water at any point
in time. Across streams large-sized sediments (> 20-mm)
were highly correlated to DRP and TP concentrations,
because attached algae can act as temporary P storage
mechanisms. As our sampled sites have a large portion of
opyright © 2011 SciRes. JEP
Physic oc hemical Characterization of Sediment in Northwest Arkansas Streams637
the substrate composed of larger-size fractions, biofilm
growth is likely a key regulator of DRP concentration.
Further research concerning biological uptake in these
streams is warranted to encompass all mechanisms which
may be of importance to P transport within these water-
sheds. Fine-sized sediment characteristics within streams
were relatively similar with sand predominating at all
sites; however, differences were apparent in the M3P
concentration of specific streams. Thus, simple substrate
composition determination of fine-sized sediments does
not implicitly determine the concentration or controls P
in stream sediments. Three transects appear sufficient (in
most instances) as a sampling strategy adequate to meas-
ure mean M3P and PSRmod content of fine-sized sedi-
ments in Northwest Arkansas streams and those with
similar characteristics. However, if large variations in
M3P and PSRmod exist, it is likely more samples are
needed to determine M3P and PSRmod. Modified P Satu-
ration Ratio was recently described by Haggard et al. [9]
as predictor of DRP concentrations in stream water. The
current research was conducted at different locations and
times than the earlier work and found strong correlation
between PSRmod and stream DRP concentrations and thus,
enhances the utility of PSRmod for varying times and lo-
cations. Further, investigations focusing on PSRmod will
enhance the understanding of its adaptability to other
sites and locations. Sediment is important in regulating
water column P and thus, in determining P transport to
lakes and reservoirs of the region. These findings in
streams of the UIRW provided an exceptional area to
investigate nutrient enrich ment in a region in which both
urban and agricultural sources operate alongside one an-
other. These findings can be expanded to streams with
similar physicochemical compositions, particularly in
regions where nutrient enrichment from agricultural
sources such as CAFOs are of importance.
5. Acknowledgments
The authors wish to give special thanks to Jason Corral,
Tony Zambrano, Stephanie Williamson, Josh Romeis,
and Tarra Simmons for assistance in field collection and
lab analysis. Also, thanks to Dr. Edward Gbur of the
University of Arkansas Agricultural Statistics Laboratory
for his help in experiment design and data analysis.
Funding was provided by the Arkansas Water Resources
Center through a USGS 104-B grant.
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