How to pass by Value in Scala

Question

I wrote this simple program ( I pass Map from String to Int to method as a parameter) and it seems that is passing it by reference. How do I make it pass by Value?

import scala.collection.immutable._
import scala.io.Source._

/**
  * Created by Alex on 5/11/16.
  */
object ScalaPassByValue {

  type Env = scala.collection.mutable.Map[String,Int]

  def mt_env : Env = {
    scala.collection.mutable.Map.empty[String,Int]
  }

  def extend_env (sym: String, v: Int, env: Env) : Env = {
    if(env.contains(sym)) {
      env(sym) = v
      env}
    else {
      env += (sym -> v)
      env}
  }

  def main(args: Array[String]): Unit = {

bar(mt_env)

  }

  def bar (env: Env) : Unit = {
println("In A")
  extend_env("a", 666,env)
    print_env(env)
    bullshit2(env)
    bullshit3(env)

  }

  def bar2 (env: Env) : Unit = {
    println("In AB")
    extend_env("b", 326,env)
    print_env(env)
  }

  def bar3 (env: Env) : Unit = {
    println("In AC")
    extend_env("c", 954,env)
    print_env(env)
  }

  def print_env(env: Env) : Unit = {
    //println("Environment")
    for ((k,v) <- env){
      v match {
        case value: Int => print("Arg: "+k+" = ")
          print(value+"\n")
        case _ =>
      }
    }
  } 
}

Ideally I want main method to pass empty map to method bar which would add a map from 'b' to 666, then call two method to add 'b' and 'c' respectively. At the end I want to get printed

    In A
    Arg: a = 666
    In AB
    Arg: a = 666
    Arg: b = 326
    In AC
    Arg: a = 666
    Arg: c = 954

but get this:

In A
Arg: a = 666
In AB
Arg: b = 326
Arg: a = 666
In AC
Arg: b = 326
Arg: a = 666
Arg: c = 954

How can I make Scala pass my Map by value so modification in call to bar2 doesn't modify original map