Table 3 should be here
3.2 Biochemical methods for denitriding
Fig. 3 showed the concentration distribution
of O2, NO3- and DOC at
typical moments of water level fluctuation under the conditions of
different surface water DOC concentrations. In the surface
water infiltration period (t <1.5d), the concentration
distribution of NO3-
(or O2) was almost unchanged in all cases. It was
because the dominant factor affecting the concentration distribution of
each solute was its infiltration capacity from surface water, by
comparison, the change of DOC concentration in surface water had
relatively small impact on AR, NI and DN during this period, namely, the
solute reactivity was much smaller than the infiltration capacity.
During the groundwater backflow period (t >1.5d),
the concentration distribution of NO
3- (or O2) varied
greatly under different cases, mainly due to the riparian zone
no longer receiving the solute recharging from the surface water, and
the chemical reaction was the dominant factor in the change of solute
concentration distribution. Comparing the different cases in Fig. 3,
with the increase of DOC concentration in surface water,
the NO3 - plume shrunk obviously in
the water level descending period, and the low-concentration-zone of
O2 expanded significantly, which indicated that
increasing the DOC concentration in surface water could strengthen the
AR and DN and thereby enhance the denitriding effect. During the whole
water level cycle, when the DOC concentration of surface water increased
from 5mg/L to 15mg/L (namely, raise twice), the denitriding amount
(M rem-NO3) increased from 6.34g
to 22.70g (namely, raise 2.6 times), correspondingly, the maximum
denitriding rate increased from 1.58g /d to 3.39 g/d. Since the total
amount of NO3– infiltrating into the
riparian zone (M in-NO3 =40.90g) was a constant,
the denitriding efficiency (N rem-NO3) increased
from 15.5% to 55.5 % (Table 4).
By comparing case 4, 5 and 6, the AR and DN processes in the riparian
zone were obviously enhanced with the increase of DOC concentration
in groundwater, which was corroborated by the decrease of
overall O2 concentration, significant increase of
low-value area of DOC concentration and obvious shrink of
the NO3- plume during
the water level descending period (concentration distribution picture
was omitted). During the water level cycle, when the
groundwater DOC concentration increased from 0mg/L to 10mg/L, M in-NO3
increased from 6.34g to 22.23g, and the maximum denitriding
rate increased from 1.58g/d to 4.89g/d correspondingly.M in-NO3 =40.90g kept no change, resulting
in N rem-NO3 increased from 15.5 % to
54.5 % (Table 4). Thus, when the DOC concentration of surface water or
groundwater increased to the same extent (10mg/L), the increase of
maximum denitriding rate induced by surface water was smaller than that
of groundwater (3.39g/d <4.89g/d), but the final denitriding
amount was larger (22.70g >22.23g). This was because that
the increase of groundwater DOC concentration could ensure the riparian
zone with a relatively high DOC concentration in a short time, thereby
improving the denitriding rate. However, the total DOC involved in the
DN process was relatively small, resulting in a small denitriding
amount.
Denitrifying bacteria mainly affected the DN process, while had little
effect on AR. Comparing case 7, 8 and 9,
the DOC and NO3- plumes involved in
the DN process decreased with the
increase of X DN, while the O2
plume had no significant change (concentration distribution picture was
omitted). During the water level cycle, whenX DN increased from 2mg/L/d to 6mg/L/d , M rem-NO3 increased from 6.34g to 11.38g,
the maximum denitriding rate increased from 1.58g/d to 4.61g/d
correspondingly, while M in-NO3 =40.90g kept no
change, resulting in N rem-NO3
increased from 15.5% to 7.8% (Table 4 ). WhenX DN increased 2 times, the denitriding amount and
maximum denitriding rate increased about 5g and 3g/d respectively. By
comparison, when the surface water DOC concentration increased 2 times,
the denitriding amount and maximum denitriding rate increased about 16g
and 1.8g/d respectively.
Therefore, increasing X DN could speed up the
denitriding process, but the denitriding process would be limited to the
infiltration amount of DOC from the surface water, causing the
denitriding amount by increasing X DN was smaller
than that by increasing the DOC concentration in surface or groundwater.