3 Natural Attenuation of Pharmaceuticals in the Aquatic …
85
HLT = ln(2)
1
τ
(3.5)
Where τ is the hydraulic residence time (d), and β is the dilution factor correction
due to flow increase from lateral flows.
Among the pharmaceuticals (acetaminophen, venlafaxine, carbamazepine,
ibuprofen, and diclofenac) studied, significant attenuation was observed for ibuprofen (3.3 h) and its metabolites (carboxy-Ibuprofen (9.2 h) and 1-hydroxy-Ibuprofen
(2 h)).
Similarly, Acuña et al. (2015) assessed attenuation of 75 pharmaceutical in four
rivers of varying physicochemical and molecular properties in different segments
with changing environmental conditions. Initially, the dilution coefficient as a result
of vertical and lateral dilution due to groundwater and tributaries, respectively, along
the river for each pharmaceutical was calculated. Both chloride and sulfate anions
were used as tracers. River attenuation was subsequently estimated by comparing
the decrease in pharmaceutical and nutrient concentrations relative to the dilution
coefficients. First-order degradation constant for each pharmaceutical in the stream
was evaluated as per the following equation.
k =
1
t
ln
C 0
C τ
(3.6)
where C τ and C 0 represent the concentration of pharmaceuticals at time τ and initial
concentration of pharmaceuticals, respectively, and τ is the travel time along the
river segment.
Furthermore, the non-dimensional mass transfer coefficient (v f ) was estimated
using Eq. 3.7 (O’Connor 1988)
v f =
Q
wv
k
(3.7)
where Q is the mean river flow rate (m
3 /s), w is the mean width (m), and v is the
mean velocity (m/s). v f is a dimensionless parameter that standardizes for river flow
rate, velocity, and river width and thus allows comparison of attenuation rates among
rivers.
The dilution corrected attenuation rate was highly variable, and this variation may
be attributed to both the physicochemical properties of the pharmaceuticals and the
local environmental conditions. There was a lack of significant correlation between
physicochemical properties and v f of individual pharmaceuticals. In contrast, previous modeling studies (Gioia and Dachs 2012) on organic chemicals demonstrated a
negative correlation between attenuation rate and volatility and hydrophobicity. The
attenuation was higher for organics having low to medium volatility (−4 < log K ow
< − 2) and low hydrophobicity (0 < K ow < 4.5). Similarly, no significant correlation
was noticed between v f and pharmaceuticals grouped based on their ring structure
85
HLT = ln(2)
1
τ
(3.5)
Where τ is the hydraulic residence time (d), and β is the dilution factor correction
due to flow increase from lateral flows.
Among the pharmaceuticals (acetaminophen, venlafaxine, carbamazepine,
ibuprofen, and diclofenac) studied, significant attenuation was observed for ibuprofen (3.3 h) and its metabolites (carboxy-Ibuprofen (9.2 h) and 1-hydroxy-Ibuprofen
(2 h)).
Similarly, Acuña et al. (2015) assessed attenuation of 75 pharmaceutical in four
rivers of varying physicochemical and molecular properties in different segments
with changing environmental conditions. Initially, the dilution coefficient as a result
of vertical and lateral dilution due to groundwater and tributaries, respectively, along
the river for each pharmaceutical was calculated. Both chloride and sulfate anions
were used as tracers. River attenuation was subsequently estimated by comparing
the decrease in pharmaceutical and nutrient concentrations relative to the dilution
coefficients. First-order degradation constant for each pharmaceutical in the stream
was evaluated as per the following equation.
k =
1
t
ln
C 0
C τ
(3.6)
where C τ and C 0 represent the concentration of pharmaceuticals at time τ and initial
concentration of pharmaceuticals, respectively, and τ is the travel time along the
river segment.
Furthermore, the non-dimensional mass transfer coefficient (v f ) was estimated
using Eq. 3.7 (O’Connor 1988)
v f =
Q
wv
k
(3.7)
where Q is the mean river flow rate (m
3 /s), w is the mean width (m), and v is the
mean velocity (m/s). v f is a dimensionless parameter that standardizes for river flow
rate, velocity, and river width and thus allows comparison of attenuation rates among
rivers.
The dilution corrected attenuation rate was highly variable, and this variation may
be attributed to both the physicochemical properties of the pharmaceuticals and the
local environmental conditions. There was a lack of significant correlation between
physicochemical properties and v f of individual pharmaceuticals. In contrast, previous modeling studies (Gioia and Dachs 2012) on organic chemicals demonstrated a
negative correlation between attenuation rate and volatility and hydrophobicity. The
attenuation was higher for organics having low to medium volatility (−4 < log K ow
< − 2) and low hydrophobicity (0 < K ow < 4.5). Similarly, no significant correlation
was noticed between v f and pharmaceuticals grouped based on their ring structure