Stream Limestone Addition

Objective:
To passively raise pH levels in surface streams affected by acid drainage.

Description:
Limestone addition into a stream is a relatively new technology for passively treating acidic waters. Limestone can be added into a stream using dump trucks filled with sand, gravel, and cobble-size limestone. Once limestone is dumped into the stream, it can be spread out, constructed into channel bars (Figures 1-3), or left for the stream to rework and redistribute the newly deposited limestone. The dissolution of limestone by acidic waters releases alkalinity, increasing pH. The increase in pH through alkalinity release may precipitate metals as hydroxides, especially ferric iron and aluminum. Refer to limestone and lime treatment and anoxic limestone drain technologies for limestone dissolution and neutralization reactions. Limestone armoring is inevitable, although it is assumed that even armored limestone will slowly dissolve. This method of generating alkalinity in a stream is passive, as the stream slowly reworks the limestone and can redistribute the limestone downstream depending on the stream’s flow regime.

                          

                                                  Figure 1. Conceptual model of limestone channel bars in a stream.
                                                  Note that water flows both around and through the limestone bars.

                                                  
                                                             Figure 2. Limestone sand addition to a creek.
                                                       Source: http://www.wvu.edu/~agexten/landrec/land.htm

 

                                                 
                                                                     Figure 3. Limestone channel bars.
                                                      Source: http://www.facstaff.bucknell.edu/kirby/LstCh.html

Climate:
Addition of limestone sand and construction of limestone bars can be a viable alternative acid drainage treatment option for any climate. However, these treatments may be susceptible to the frequency and intensity of peak flows.

Treatment Process:
Limestone sand is the most often used grain size as smaller sized particles have a larger reactive surface area. This allows for more surface area for dissolution, even if the limestone particle becomes armored with ferric and aluminum hydroxides. Agitation and scouring of limestone particles by steam flow can help to keep fresh surfaces on the limestone particles available for dissolution.
If sand sand-size limestone particles are added to a stream, the sand is spread throughout the stream channel or the sand is left in place to allow the stream to rework and redistribute the sand in a passive manner.

Limestone channel bars are constructed with combinations of sand, gravel, and cobble-size limestone particles. The channel bars are usually arranged in a manner that increases stream tortuosity, effectively increasing the interaction between the acidic water and the limestone bars (Figure 1).

Requirements and Limitations of Use:
It is recommended that the use of limestone particles in stream systems to raise pH is limited to streams with low to moderate acidity and low dissolved metals (particularly ferric iron) (ref). Both limestone sand and limestone channel bars can raise pH to above 6.0 (Zurbuch, 1996).

Precipitation of dissolved metals on the stream bed, especially ferric (ppt if pH > 3.5) and aluminum (ppt if pH > 4.5) hydroxides, have been commonly observed at locations where limestone has been used to raise alkalinity. These metal precipitates can become remobilized if limestone is not replenished on a certain schedule (2-3 years) and sources of acid drainage continue to add acidity to the stream.

Limestone channel bars require more initial work to construct but may remain in the stream longer, as the larger size of the limestone particles takes a longer time to dissolve.
The arrangement of limestone channel bars is dependant on stream properties such as gradient, stream flow including peak, average, and minimum flow, and stream acidity.
Increasing the tortuosity of a stream to influence limestone reactivity can be valuable, but emphasis is needed to ensure that peak flows do not scour and wash out the gravel bars. Sand-sized particles can be suspended during flood periods and deposited in the flood plain out of the active channel, resulting in a loss of available limestone for alkalinity production.

Calculation of Stream Acidity and Limestone Amounts:
Dosage of limestone sand used for raising stream alkalinity is somewhat subjective. However, most methods base the limestone dosage according to the amount of stream acidity produced in a year. Stream acidity can be calculated for a specific reach of a stream (as long as acidity inputs are not located down steam) or total acidity may be calculated as the summation of acidity inputs from individual tributaries affected by acidic drainage.

Limestone dosage is dependant on the schedule of limestone addition. If the schedule is every two to three years, two to three years worth of limestone will need to be added to the stream. One dosage method is to initially add 3 to 4 times the amount of limestone needed to neutralize stream acidity for one year. The following years, the dosage of limestone can be equal to a stream’s yearly acidity load.
Stream acidity is determined from acid-base titrations of water samples collected from a stream (for a review of acid-base chemistry see Langmuir, 1995). Then, acidity of the water sample is multiplied by the flow rate.

Total alkalinity determination based on stream discharge

Acidity Load = Moles of CaCO3 equivalent acidity per minute
                  

Example
Stream flowing at a rate of 20 cfs with 75 mg/L acidity
                

Limestone
              

Correction factors may have to be used for the total amounts of limestone. For example, a 90% dissolution rate is used for high quality limestone. Limestone dissolution may be less efficient in stream systems where metal hydroxides armor limestone particles.

Predicted Performance:

Synergistic Technologies:
Conventional lime and limestone treatment
Diversion Wells
Open Limestone Channels
Anoxic Limestone Drains

Costs:
Limestone sand is $25/ton including delivery (Zurbuch, 1996). Refer to anoxic limestone drain technology for prices of crushed limestone used for channel bar construction. Channel bar construction also requires heavy equipment.

Case Studies:
Zurbuch, P.E., 1996. Early results from calcium carbonate neutralization of two West
irginia rivers acidified by mine drainage. In: Proceedings of the 17th West Virginia Surface Mine Drainage Task Force Symposium, April 2-3, 1996, Morgantwon, WV.

References:
Zurbuch, P.E., 1996. Early results from calcium carbonate neutralization of two West
irginia rivers acidified by mine drainage. In: Proceedings of the 17th West Virginia Surface Mine Drainage Task Force Symposium, April 2-3, 1996, Morgantwon, WV.