Alternative to mutable data structure in clojure

Question

I developed a function in clojure to fill in an empty column from the last non-empty value, I'm using flambo which is an apache-spark wrapper using clojure for some of its functions

(:require [flambo.api :as f])

(defn replicate-val
  [rdd input ]
  (let [{:keys [ col ]} input
    prev-col-val (atom [])
    result (f/map rdd (f/fn [ row ]
                                (if-not (s/blank? (get row col))
                                  (do
                                    (swap! prev-col-val assoc 0 (get row col))
                                    row)
                                  (assoc row col (get @prev-col-val 0)))))]
result))

I don't really like the idea of mutating prev-col-val to keep track of state, Any ideas on how to refactor the above to preserve clojure's immutable data structure?

Input is of the form:

[["04" "2" "3"] ["04" "" "5"] ["5" "16" ""] ["07" "" "36"] ["07" "" "34"] ["07" "25" "34"]]

and desired output is:

[["04" "2" "3"] ["04" "2" "5"] ["5" "16" ""] ["07" "16" "36"] ["07" "16" "34"] ["07" "25" "34"]]

Thanks